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.