Update file_line resource to support 'after'.

When adding new lines to a file the 'after' option can be useful
when you need to insert file lines into the middle of a file.

This is particularly helpful when using file_line with sectioned
config files.

NOTE: the after option only works when adding new lines. If you are
updating an existing (matched) line it will simply modify it in place.
This assumes it was in the right place to begin with.
(maint) Improve the tests and readability of file_line

Without this patch the implementation of the file_line provider is a bit
convoluted with respect to the newly introduced "after" parameter.

This patch addresses the problem by separating out the concerns of each
case into their own methods of handling the behavior with the match
parameter, handling the behavior with the after parameter, or simply
appending the line.
Merge pull request #174 from dprince/file_line_after

Update file_line resource to support 'after'.