Add system-dependent mode

Just an idea that I had in mind

In "system-dependent" mode we can suggest/activate bears based on the existing bear dependency installed on the user's system. For example activate CPPCheckBear if cppcheck is already installed in the system.

On the other hand, we'll still have a system independent mode which assumes nothing about the system and can be useful in using quickstart remotely.

status/needs-design

@satwikkansal, it looks like this is a dependency for parts of the green project. Brute force cant work with a bear that has missing deps.

hmm, interesting.

I think I see two ways of using green mode.

• Green mode + system-dependent mode: Useful for users who are trying out coala for the first time and need minimum friction to get going.
• Green mode + cib (system-independent mode): We can use this as GCI tasks, by creating green .coafile for projects that can potentially benefit from using coala, making a PR to their repo. The dependencies for the green coafile can be installed using cib. Related #137

The later use-case is useful is green-mode ends up taking a lot of time to generate coafile (we can't expect users to spend that much time)