Tweak Match and MatchExactly so they can be used for regex on files

Fix Should Contain and Should ContainExactly
* You should be able to specify more than one line for the test, as in `| Should Contain "line`nanother"`
* You shouldn't have to think about RegEx in a "contains" test
* You should be able to do: `"Some string" | Should Contain "string"`

After doing that, I also tweaked Match so it could be used on file content if you pass a FileInfo object (not just a path): `get-item TestDrive:\Test.txt | Should match "the contents"`