Skip filename pattern check for provided files

By default, when discovering files for users, we use the filename
patterns to determine whether or not we should check that file. However,
when a user provides the path to a file, we should instead skip checking
the name against the filename patterns provided.

For example, in Flake8 2.6 this worked:

    $ flake8 bin/script.py
    $ flake8 bin/script

But prior to this commit only

    $ flake8 bin/script.py

works. This commit will skip the filename pattern check if the user
provides the path explicitly which allows

    $ flake8 bin/script

to work again as expected.

Closes #266