Add a --language (-l) option to the formatter delete commands in order to...

Add a --language (-l) option to the formatter delete commands in order to allow removing formatters from language categories

This is slightly harder to test because formatters cannot be added to language categories, so deletions are irreversible (in a debugger run)
I plan to add a test case soon, but I need to think about the right approach to obtain one

llvm-svn: 251660