fix can't repeat if ':' map to something other key

It can't be repeated if ':' map to something other key.
Because repeat.vim use feedkeys function for repeat and
if ':' key map to another key, execute another key instead of ':'.

Like below

call feedkeys(":SplitjoinSplit\<cr>")
                          ^
                          ':' execute as another key
Merge pull request #21 from deris/fixed/cantrepeat

fix can't repeat if ':' map to something other key