Implement live interactive mode (#15)

This commit adds LiveEasyAlign command. (The name was chosen not to introduce
ambiguity when typing in only the prefix of the command: e.g. `:EasyA*|`)
In live interactive mode, the selected text is aligned on-the-fly as the user
type in. In order to finalize the alignment, the user has to type in the
same delimiter key again. (Or CTRL-X on regular expressions)

test/commandline.vader

@@ -16,6 +16,7 @@ Given (fruits):
 
 Execute (regular expression):
   %EasyAlign/[:;]\+/
+  AssertEqual ':EasyAlign /[:;]\+/', g:easy_align_last_command
 
 Expect:
   apple ;:;; banana::cake