Without calling start and stop, our output file is not used by our
Formatters and any formatting plugins.

Related #180

In Flake8 2.x we allowed people to use # noqa at the end of a multiline
string to ignore errors inside the string (e.g., E501). Being blissfully
ignorant of this, I never accounted for it in Flake8 3. This fixes the
oversight and allows multiline statements to have the # noqa at the end.

Closes #177

It is possible to write plugins which are only a function. At the moment they
are called on each line manually. This allows the function also to be called
on each file once. It works similar to creating the class and calling `run` on
it immediately. The plugin function needs to return a generator.

This is based on the original comment in the `FileChecker.run_ast_checks`
method, but slightly modified as the original comment would've called the
return of the function. But the function could return the reports directly.

In the case where alternate separator is None, we use '' which will
always be in any string. We want to skip that case.

Also we only run our tests on AppVeyor, not all of our testenvs.

When normalizing paths, we want to handle the following cases:

- Someone is using a Windows-style path on Windows
- Someone is using a Unix style path on Unix
- Someone is using a Unix style path on Windows

os.path.sep will handle the native directory separator character while
os.path.altsep (when set) will handle alternate separators. Further,
os.path.abspath does the right thing on Windows when handed a Unix-style
path.

Related to #175

The documentation for the `FileProcessor` class used `indect_char` while the
class itself uses the more sensible name `indent_char`. This updates both the
docstring as well as the documentation.

This simplifies the changes, reduces the scope of refactors apparently
for refactoring's sake and ensures that the internals are reasonable.

It also airs on the side of preserving information rather than
discarding or overwriting it.

This way plugins like flake8-putty can have access to the correct filename.

For the sake of IDEs, check filename for exclusion even if the file is directly
named in the command line.

Also, if the filename is "-" (stdin) check the provided display name for
exclusion.

Also, avoid calling path checking functions on the "-" filename:

 * fnmatch.fnmatch()
 * os.path.isdir()
 * os.path.exists()