Colorize file as well as adb output

If passed one argument which is a file then logcat will read/colorize
that files contents and write to stdout. Otherwise will behave as
before by colorizing output of "adb logcat".

Note, I wanted to also support reading from STDIN, but didn't see a clean way of doing this. Ideally I'd like to run logcat in one of three ways:

1. logcat // reads stdout of "adb logcat"
2. logcat saved_output.txt // Colorize saved_output.txt to stdout
3. cat saved_output.txt | logcat // Colorize stdin to stdout

This change adds #2, and I'll do #3 when I have the need (or when asked).