ability to switch color theme

[astavonin]

The submission introduces new flag theme. There are to possible values: dark (default value, current helloworld theme) and default (theme based on terminal settings).

[divan]

Hi Alex, thanks for PR and sorry for the response delay

I initially dumped themes' support in favor of only one cause it would require some efforts to make UI looks great with all themes (choosing default colors, mainly), and TermUI currently doesn't provide clear theme framework.
Taking into account there are only two themes available, I decided it's worth it at all.

Anyway, if there is a demand for it, I don't mind introducing support for themes in expvarmon, but adding new flag option doesn't look justified for me. I doubt this will be used a lot.

So, what do you think if we move configuration part into environment variable, say "EXPVARMON_THEME"?

[astavonin]

Personally, I prefer to use key. It bit more flexible and explicit as you always knows your flags and able to change it easily.