shell commands: colourize messages

[EricFromCanada]

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Adds colour to shell-based commands via a new ohai function, and adds a check for HOMEBREW_NO_COLOR where necessary.

[issyl0]

What benefits does this have?

[EricFromCanada]

Consistency with .rb-based commands and readability. It's something I noticed, as one who has several VM windows open for testing and runs brew update-reset frequently.

[issyl0]

Makes sense - thanks!

[MikeMcQuaid]

Nice work here @EricFromCanada, I like it!

[MikeMcQuaid]

Would be good to also use colours unconditionally if HOMEBREW_COLOR is set (but that can be a follow-up PR).