Non-existant linter list

[jayvdb]

I think it would be useful to maintain a list of "missing" linters, where we know about a problem, and do not know of a tool which checks it.

Where reasonable, we could suggest they could be solved by creating a coala native bear.

For others, we could recommend an existing linter that would be a 'natural fit' for the extra functionality, and create an enhancement request in the external project, and track it.

But for most of the missing linters, we would be more interested in someone creating an external linter, and contributing a bear.