Fix/Update bash language

This patch includes the following improvements to the bash language:

* Remove dependency on `clike` because it has almost nothing in common
  with it.
* Add some missing keywords and builtin functions
* Add support for switch statements
* Add support for arithmetic environments
* Allow +-*/ only in arithmetic environments
* Fix hashbang syntax
Do you want to highlight the rest after the shebang? If not, you could just use `/^#!.*$/`

I didn't find anything specific, but I think the shebang must start at the beginning of the complete input (and especially no whitespace before it).