Add support for configurable engines path

[jsyeo]

This PR is to add support for scanning engines that are not on the engines/ path.

This is the first step needed to fix #664. See #664 (comment).

The next step would probably be adding an option so you could point it at the engine directory - that would allow for pointing it at gems too.

This PR adds an engines_path option to brakeman to allow users to add engines that the application uses. The path to the engines can either be absolute or relative to the project root. For example, if the brakeman.yml config file contains this:

:engines_path:
  - engines/user_removal

Brakeman will scan the engines/user_removal directory relative to the project root. If the path starts with a forward slash, we assume that the path is relative. Like so:

:engines_path:
  - /path/to/rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/administrate-0.1.4

[jsyeo]

This is the first step needed to fix #664.

I guess the next step needed would be to fix #848.

[presidentbeef]

Needs to be added as an actual commandline option :) in https://github.com/presidentbeef/brakeman/blob/master/lib/brakeman/options.rb

[jsyeo]

@presidentbeef added. :)

[presidentbeef]

Hi Jason, sorry for the delay on this. Committing to reviewing your pull requests tomorrow (Friday).

[jsyeo]

Thanks! No worries. 😉

[presidentbeef]

The engines directory should be included by default. For testing, I would add a non-standard engines directory to rails4_with_engines.

Additionally, the option should be able to specify the top of the directory (e.g. engines not just engines/some_engine). I think this would already work, but just pointing it out since your test case does not work that way.

[presidentbeef]

Merging with #948