Add support to caseInsensitive option

[LucasZastrow]

Pleasse consider the possibility of adding support to caseInsensitive option.

https://github.com/antlr/antlr4/blob/master/doc/options.md#caseinsensitive

[lextm]

You should migrate to Antlr4BuildTasks and Antlr4.Runtime.Standard for ANTLR 4.7 and above,

https://github.com/kaby76/Antlr4BuildTasks/blob/master/Readme.md