cmd: respect TERM=dumb by not using colors

[acln0]

Description

Check for TERM=dumb when determining whether to use colors when printing.

Fixes #510

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Version of Golang used when building/testing:

• 1.11
• 1.12
• 1.13
• 1.14
• 1.15
• 1.16
• 1.17
• 1.18
• 1.19

How Has This Been Tested?

Per #510, I tested by inspecting the output, which no longer contains colors.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[LandonTClipp]

I'd prefer this to be pulled in as a config value that can be bound to an env var like MOCKERY_DISABLE_COLORING.

[4ad]

TERM already is an env var -- the right one for this case.