Fine-tuning http-plugin evaluation

[flandweber]

The argument --valid-http-contains was added, which restricts what is accepted as a valid response by the webserver.

Additionally some minor syntax changes were made.

Background

Some sites, such as google and facebook, respond with html documents containing error messages if a request was malformed. This leads to geneva misinterpreting this response as a successful surpass of the censorship.

Solution

The evaluator checks wether a specified word (which we know is contained in the uncensored site) shows up in the servers response. If not, the strategy is punished like if it broke the connection (which it did).

It works analogous to --injected-http-contains.

[Kkevsterrr]

Looks great - thanks for your contribution!