Add support for config files in per_file rule

[furtib]

Why:
We want to support config files inside the per_file rule, too.

What:
Added the config rule to the per_file rule, based on the codechecker_test rule.

Addresses:
#30

[furtib]

You can leave out the platform attribute by removing these lines (see above)
I think these are necessary to support cross-platform analysis (?) (is there even such a thing?)

[Szelethus]

I had a change of heart wrt this patch. I think we should absolutely add a query to CodeChecker we don't need to sort out how its configured, but for now, this is an acceptable fix and a small part of the patch.

[Szelethus]

The patch is now much simpler. Nice! Please create an issue for our poor ctu detection before merging.

[furtib]

We should know that during analysis phase, we cannot read from files (e.g. see if CTU is enabled in a config file), and also we cannot query other programs (e.g. CodeChecker cmd query-feature ctu)
We may have to put a restriction on the use of the CTU flag in the config file.