Implement support to variable STREAM_INPUT_BODY inside libmodsecurity

[zimmerle]

More information about STREAM_INPUT_BODY can be found at ModSecurity reference manual: https://github.com/SpiderLabs/ModSecurity/wiki/Reference-Manual

[zimmerle]

As of now, we don't have a proper way to handle the content in a stream manner. It won't be different from use others variables that represents the same data. Therefore I am marking this as won't implement. At least for this next version.