Allow hiding warnings when shell enviorment variable is missing

[dbrodie]

I want to use the shell environment variable module to show when the shell is inside tab-rs and some other similar examples. The use case for that is, that for the shellenv module to show the segment if the variable exists and not if it doesn't. I wouldn't mind implementing this, would just need to understand how you wont to implement it.

• Configurable? Have an argument to specify if to print a warning if the environment variable, or just ignore and no warning
• What to display if the environment variable is set but empty?

IMHO, it should not be configurable, and should display an empty section when empty (like a coloured separator but without content). And should only hide the segment when unset.