Add multi-line support to pre/post_install (fixes #1890)

[rasa]

No description provided.

[rasa]

@r15ch13 Should I mod the PR to:

iex (@($pre_install) -join '`r`n')
...
iex (@($post_install) -join '`r`n')

[brandon93s]

+1 for \r\n - consider the use case of an if statement which wouldn't work with ;.

The current implementation is a bit easier to understand to me, but my ps is pretty weak. Checking for an explicit array instead of not string would be even more clear.

[stkb]

To explain, the "array subexpression operator" (@( )) does the check for an array already. It's like a "ensure array" operator. If the value passed in to it is already an array, it leaves it as it is. If it's not an array, it creates a singleton array from it. From there it's just a -join to ensure we always have a string again.