case-insensitive matching and look-behind

Support for:

• lookbehind positive (<=abc) and negative (<!abc)
• case insensitive matching for ASCII only ctre::OPERATION<"regex", ctre::case_insensitive>(subject_or_range) (example: ctre::match<"[a-z]+", ctre::case_insensitive>(input)

Notes:
don't use mixes case range when doing case_insensitive:

• [a-z] will match a-zA-Z
• [a-Z] won't match anything (as ascii(a) > ascii(Z))
• [A-z] will match more than you expect A-Z[\]^_a-z + backtick (as I don't know how to type in markdown correctly)