Pass the actual feature file to the IStepDefinitions instance

This can be useful in cases where the lookup of step implementations
wants to consider the directory of the feature file for finding steps
instead of going through a complete project. In particular to avoid reporting
steps that are not usable from a certain feature file due to them being
in unrelated directories.
Merge pull request #79 from apaku/pass_feature_file

Pass the actual feature file to the IStepDefinitions instance
Implement a basic syntax checker with error marking

This uses the Parser's syntaxError which is triggered for cases like
a step following the feature line directly or having more than one feature
in a single file.

Unfortunately its not usable to detect lexical errors since the parser
currently throws an exception without metadata for such cases (just
starting to write arbitrary text on the beginning of the file).

The updating is done on save for now since thats easier to hook up than
doing it directly after typing.

Potential improvement could be add warnings-support for steps that have
no matching definition.

fixes #22
Merge pull request #78 from apaku/mark_errors

Implement a basic syntax checker with error marking