Fixes #153

This reverts the first attempt (commit
8310b029), because relative dates in terms of
hours were printed.

This attempt passes the now object that we just created to humanize(), so that
will be used as a offset. If we don't supply that object, arrow will create a
different datetime object representing 'now', which might be slightly off
w.r.t. the now object we created.  Therefore the floating point arithmetic done
within the arrow library might not produce the desired results (delta / 86400).

By passing the offset ourselves I was able to produce reliable
results.

With the previous method of using arrow, the same date object would
result in different humanized dates. Apparently using now() and then
replacing the date components triggers some inaccuracy.

Simplify the call to arrow by passing it the datetime object directly.

Otherwise it's hard to test the output when the order differs on every
execution.

Also improve the logic to prevent updates for changes caused by the
current topydo instance. This was done to ignore the first file update
event after writing out the file, and trigger updates for all other
file updates that have to be external.

The error message does assume you have installed topydo through pip,
which I expect to be usually the case.

Fixes issue #150

This is applicable for prompt mode and column mode. Changes are
immediately read whenever todo.txt is changed.

Current issues:

* Writes caused by the current topydo instance also trigger the file
  update: needless overhead.
* Column mode: items are not immediately redrawn on update

This mostly addresses issue #142.

On Travis CI, the pypy3 tests may fail due to concurrency issues,
probably because of the usage of (shared) temporary files.

So, limit the number of processes used by green to 1 when pypy3 is used.

This implies that failures for pypy3 are no longer allowed.

Too many (random) failures.

Support user-specified column-layout file

User can now use '-l' option with an argument *after* 'columns' command
and topydo will try to load column layout from that location. If it
won't find specified file it will use standard column-layout file
locations as a fallback.

Example of use:
Will work: `topydo columns -l ~/fooBar_columns`
Won't work: `topydo -l ~/fooBar_columns columns`

Implements #147

That way the filetype detection of the todo.txt vim plugin is triggered and shows the appropriate syntax highlighting.

https://github.com/freitass/todo.txt-vim

Fix importance being wrong when due date is a distant monday.

Close temporary file after saving to it

This can solve the problems with file handle being occupied by python
while editor is trying to save user changes from `topydo edit` on some
operating systems.

Potential fix for #141

Add HiddenTagFilter

Properly unmark items when resetting state

After introducing cache for TodoWidget objects, widgets highlighted with
'mark' action were not unmarked after 'reset' action and they reappeared
highlighted ("marked") after reverting `do` or `delete` commands.

This serves two purposes:

* Relative dates and progress colors are shown correctly on the next day
* Stale cached items get wiped at least once a day (for those who continiously
  run this mode)

When editing one or more todo items, some items may not have changed, therefore
having the exact same source text. The EditCommand still deletes the Todo
instance and creates a new one and puts it in the todo list. But the widget
cache only looked at the source text, found a widget from before the edit and
will use it. But the widget still has a reference to the old Todo instance.
When executing an action on it (e.g. mark it as complete), the TodoList doesn't
remember that todo item, which results in a crash.

Fixed by updating the reference to the latest Todo instance when the sources
are equal but the instances are not.