Add support for a 'match' parameter to file_line

[cprice404]

This commit adds a new parameter called "match"
to the file_line resource type, and support for
this new parameter to the corresponding ruby
provider.

This parameter is optional; file_line should work
just as before if you do not specify this parameter...
so this change should be backwards-compatible.

If you do specify the parameter, it is treated
as a regular expression that should be used when
looking through the file for a line. This allows
you to do things like find a line that begins with
a certain prefix (e.g., "foo=."), and *replace
the existing line with the line you specify in your
"line" parameter. Without this capability, if you
already had a line "foo=bar" in your file and your
"line" parameter was set to "foo=baz", you'd end up
with both lines in the final file. In many cases
this is undesirable.

[cprice404]

This is a prerequisite for the postgres module that I'm working on. Would love to get a quick review from any or all of @jeffmccune , @glarizza , @rcoleman , @nanliu , @bodepd