Add tcomment plugin

Trying it out to see how it compares to commentary.vim. Getting annoyed
that commentary.vim doesn't know to use JavaScript comments for a script
embedded in HTML.
Remove commentary.vim plugin

Tried tcomment which is able to recognise the code and apply the
appropriate comment instead of blindly using the comment format
specified for the current file type. Removing commentary.vim as part of
the switchover.
vimrc: remove commentary.vim autocmds

These are no longer required now that I've switched to tcomment.