Export function to manipulate GitPromptSettings

[dahlbyk]

Spawned from #256 (comment), similar to #111.

Pros:

• Discoverability (module definition, etc)
• Tab completion
• Parameter Validation (e.g. BranchBehindAndAheadDisplay "enum")

Cons:

• More code to maintain
• Migration path from $GitPromptSettings? Or does function just manipulate the global?

[rkeithhill]

Pertinent to this discussion (and the discussion on module install script) - PoshCode/PowerShellPracticeAndStyle#79

[rkeithhill]

You get tab completion/Intellisense with the $GitPromptSettings variable so that really isn't a "pro" for a function.

[rkeithhill]

For 1.0.0 we're going in the direction of a more structured, strongly typed GitPromptSettings object. If you disagree or think this needs to be reopened, please reopen it.