Add commentstring setting to ftplugin. Enables tComment to work out of the box with vim-emblem.

vim-emblem does not work properly with the tComment plugin because it does not set the `commentstring` option, which tcomment uses to define a comment style in the absence of an explicit call to tcomment's [`tComment#DefineType#`](https://github.com/tomtom/tcomment_vim/blob/master/doc/tcomment.txt). A line is added to ftplugin/emblem.vim to solve this problem.