Modify SCC parser's show/hide detection logic.

- Find exact frame within a line that a show or hide command is sent.
- Allow an initial show prior to any text being buffered
- Fix existing test case.  Test case was trivially passing b/c no parsing
  was occuring (missing tab character is regex.)
ScenaristClosedCaptions:36
````
private const string _timeCodeRegEx = @"^\d+:\d\d:\d\d[:,]\d\d\t";
````
The tab is mandated by the spec.  As noted in the pull request comments, the test was trivially passing b/c the regex wasn't matching.