Print a warning about .clj files that are not linted, and why they are not

[jafingerhut]

My dolly project's command 'lein dolly ns' warns about files that are not analyzed because no ns form was found. Eastwood should do something similar, and perhaps also mention that .cljs and .cljx files are currently ignored, if any are found.

[jafingerhut]

Related to how issue #101 is handled -- if an option is created to try to lint .clj files that are by default ignored, obviously we shouldn't warn about them then, but maybe will want to print out some message about how they are being handled differently.

[jafingerhut]

These should be reported as a separate lint warning map for each such file, similar to other linters, for easier handling by potential code that post-processes warnings somehow. They should have their own linter name, which can be disabled if one doesn't want to see these warnings.

Similarly, the current filename / namespace warnings should be reported as warning maps, too, with their own linter name, but in that case they cannot be disabled.

[jafingerhut]

Added with this commit: a3d7354