Add evil-escape-case-insensitive-key-sequence option

[spatten]

This PR allows you to type your escape-key sequence ignoring case. So you can type "df", "Df", "dF" or "DF" and still escape, thus allowing you to escape if you're slow at releasing the shift key like I am.

I think it answers this issue: https://github.com/syl20bnr/evil-escape/issues/72 and some of the requirements/need in this issue: https://github.com/syl20bnr/evil-escape/issues/64

I'm pretty-much a beginner at Elisp and Lisp in general, so please feel free to make or request changes.

[ChauhanT]

This looks great! Is there a reason why it is not merged upstream yet?