Add a <Plug> map for users

[graywh]

If the user hasn't mapped to <Plug>MakeGreen, use <Leader>t.

[5long]

It seems that the <Plug> mapping entry and <leader>t have been remove intentionally in #11. It's just the README that falls behind the code.

[graywh]

@5long I submitted this PR before the other one was accepted.

There wasn't a <Plug> map before--this PR adds one. It makes it easier for users to make their own map instead of the old default <Leader>t.

[5long]

My bad for not noticing the time difference. The README must have tricked me.

Since there's now a :MakeGreen command, It's not that hard to map to it:

nmap ,t :MakeGreen<CR>
nmap ,t <Plug>MakeGreen " not too much of a difference IMHO
nmap ,r :MakeGreen %<CR> " Not easily achievable with mapping since mapping to <Plug> can't pass arguments.

But I do agree that adding back a default mapping would be easier for starters to begin with. And I suggest doing it less obstructively(i.e. don't override user's <leader>t) with <unique> like this: https://github.com/wincent/Command-T/blob/341d0e54822c72f32be1664f8bedc0cdbd2f87c0/plugin/command-t.vim#L36-L38

Besides, you might want to remove the trailing parentheses when mapping to a <Plug>. Example: https://github.com/tpope/vim-surround/blob/02199ea0080d744ec76b79d74ce56d51d25cf7ae/plugin/surround.vim#L561

[graywh]

The trailing () is a typo.