introduce case-insensitive version of str and match atoms

[taichi-ishitani]

There are some cases that str and match atoms would be case-insensitve.
For exmaple, parsing hexadecimal number:

(str('0x') | str('0X') >> hex_digit.repeat(1)

I think case-insensitive version of these atoms are convenience for this kind of case.
This PR is to introduce these atoms named istr and imatch.