Increase code coverage for termshark to 80%

[pocc]

I might be missing something, but I'm not seeing test functions in termshark.go. It looks like there's a smattering of unit tests scattered throughout other files, which is good. More unit tests for the main file would be helpful here (and goes hand in hand with #20).

[pocc]

Changing title to increasing code coverage. Currently, it looks like code coverage is 23%.

bash-5.0$ go test -coverprofile=c.out ./termshark
ok  	_/Users/rj/code/termshark	0.303s	coverage: 23.2% of statements