Add IGNORECASE ability to Token Remapping.

[manux81]

With reference to:
dabeaz#62
I've found a problem with tokens remapping in the eventuality of:
reflags = re.IGNORECASE
This change allows creating special cases ignoring capitalization in the eventuality of
reflags = re.IGNORECASE assignment in Lexer class.

Example:
class MyLexer(sly.Lexer):
reflags = re.IGNORECASE
# Base ID rule
ID = r'[a-zA-Z_][a-zA-Z0-9_]*'

# Special cases
ID['if'] = IF
ID['else'] = ELSE
ID['while'] = WHILE
# Now if,If,IF,iF are handle as the same special cases