Add g:rbpt_bold option to make colored parentheses bold.

Default is off.

I had trouble seeing parens when they were colored; making them bold
helps a lot.
update docs
update docs
Merge remote-tracking branch 'joehillen/bold'
update docs
Fix problems when spellcheck is on
Merge branch 'master' of github.com:boris-petrov/rainbow_parentheses.vim into boris_petrov

* 'master' of github.com:boris-petrov/rainbow_parentheses.vim:
  Fix problems when spellcheck is on