User can now specify key-shortcuts in main topydo config file under
'column_keymap' section in form of:

`<SHORTCUT> = <ACTION>`

Two main types of action are supported:
- built-in (one of: 'home', 'end', 'up', 'down', 'postpone', 'postpone_s'
  and 'pri')
- topydo commands (aliases included). Prefixed with 'cmd'. Commands to
  call on selected todo item should contain '{}' placeholder to mark its
  place in final command call in similiar fashion as in aliases
  definitions.

postpone, postpone_s and pri shortcuts are sort of prefixes for
arguments for respective topydo commands triggered on selected todo item:
- `postpone<COUNT><PERIOD>` will translate to `cmd postpone {}
  <COUNT><PERIOD>`. postpone_s will do the same but with '-s' flag
  added.
- `pri<PRIORITY>` will translate to `cmd pri {} <PRIORITY>`

Default config as an example:

```ini
[column_keymap]
gg = home
G = end
j = down
k = up
d = cmd del {}
e = cmd edit {}
u = cmd revert
x = cmd do {}
pp = postpone
ps = postpone_s
pr = pri
```

- pp23d will postpone selected item by 23 days
- ps1m will postpone selected item (threshold date included) by 1 month
- prz will set priority of selected item to (Z)