Fix source output

* use `arg[0]` to base suggestions on, so input filtering
  does not clobber the display of suggestions;
* default to `<cword>` if nor argument is passed;
* notify user if no input word is found;
* clean up source settings.
Clean up formatting

* 2 spaces soft tabs indent:
* remove function fold markers.
Remove default spell suggestion number limit

`g:unite_spell_suggest_limit` can still be set, but the default is
to leave it unset and return the same number of results as Vim.
Add source description
Set default action to 'yank'
Formatting cleanup, added modeline
Add Unite display string
Robustify functions