Add setting to point to coala executable inside the venv

[EverWinter23]

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This will comply with coala.io's guidelines to install coala inside a virtual-environment.

[RohanVB]

Temporary fix to this is installing coala globally.

This might also be related to: https://github.com/coala/coala-atom/issues/45, it's a straightforward solution, but we should probably have it by default instead of making the user do the work if the filepath doesn't match.

[Rotonen]

People using buildout will appreciate if you can also specify a project relative path like $PROJECT/bin/coala.