More flexible classmethods.

Since a parser might require a certain number of parameters, it is more
flexible to pass all of them to `_is_parser` classmethod.
The same with `is_mine`.

For instance, Skipfish has two files to represent a report instead of only one.
[skipfish] Fix support of Skipfish.

Skipfish still uses the AbstractParser class because its reports are really
specials. They consist of two .js files.
Therefore I don't think it is wise to create a special JSParser for that.
[doc] Update report and parser doc.
[parser] Fix typo.