Improve tamper scripts structure about sqlmap

[bdamele]

After further consideration, I have decided that we can close this ticket because:

• At the moment there is no need to pass parameter name to tamper script (this was needed only for HPP bypass, but as of its different WAF bypass technique, it has been implemented as a standalone switch).
• Passing headers to tamper script - this can be done already because tamper scripts support **kwargs now (they're not considered at the engine level though, so if this feature request arises, it will be implement if it brings any benefit).
• Automate selection of tamper script(s) - this will be addressed in issue #13.
• **kwargs supported across all tamper scripts.