Improve ignore

[jimivdw]

• ignore now works for multi-line regexes
• mutations are now ignored also when they cover only a part of the ignored code, or more than just the ignored code
• re-implemented discardReplacements as ignoreReplacement, which can be used to ignore certain mutation replacements (I didn't even know we could do this)
• Ignored replacements now also count as ignored mutations
• 'use strict' is now ignored by default (as requested in #18)
• Added mutateStrictModeKeyword config option, in case 'use strict'; must be mutated
• Added documentation for all this in README

[paysdoc]

I'm happy with your changes but there are merge issues which I'd rather you dealt with. So I've assigned it back to you.

[jimivdw]

Accidentally made this a pull request to master, but should have been to v1.1. I'll create a new one...