Add --output flag

[stiliajohny]

Hi, great work,

Have you consider adding an --output flag so the filename is not defaulting to out.png

[HeavyWombat]

@stiliajohny Thanks for the issue. I am terrible slow with responding to issues in github.com/homeport these days. So I apologise for the very late response here. It should be fixed with #24 and released in https://github.com/homeport/termshot/releases/tag/v0.2.2. The name of the flag is --filename. Now that I think about it, maybe --output would have been better.