commands: add new generic --skip-if command line options

[phlogistonjohn]

Depends on: #119

Previously, sambacc had the --skip-if-file= command line option that would skip running a command if a named file exists. I have a need for doing something similar for environment variable contents. In order to avoid needing to add a plethora of new --skip-if-x type options over time, add a more generic --skip-if= option and have the value contain an initial prefix like file: or env: that determines how the rest of the value will be parsed and how the skip will be determined (or not). Deprecate --skip-if-file.

Maybe it's a case of over engineering, but at least it's easy to unit test.

Examples:

• --skip-if=env:CHAT==yes, if the environment variable CHAT contains a value equal to "yes" the command will be skipped
• --skip-if=file:/foo/bar if the file /foo/bar exists the command will be skipped
• --skip-if=file:!/foo/bar if the file /foo/bar does not exist the command will be skipped
• --skip-if=env:MODE!=777, if the environment variable MODE contains a value not equal to "777" the command will be skipped
• --skip-if=always:, always skip this command (mainly to be used for testing/hacking on things)

[phlogistonjohn]

cc: @avanthakkar - for an additional review & peek into this code base

[dpulls[bot]]

:tada: All dependencies have been resolved !

[phlogistonjohn]

I noticed a stray debugging print statement was still in the unit test file. It's now removed.