fix how `prefix` is inserted into output

[alxndr]

The line can be a multiline string. This commit changes how the prefix is inserted to use a multiline regular expression instead of prepending one to line.

Fix for the bug reported in #17

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This change is 

[alxndr]

Hrm, now I'm seeing double-prefixes...

[M-Zuber]

Ping on the issue of seeing double [test].

[M-Zuber]

The issue was related to https://github.com/mochajs/mocha/pull/2401. I implemented a more complete fix, but will only be able to push it up tomorrow or after the weekend

[M-Zuber]

So basically after fighting with this a whole bunch, it would seem that our options are either no new lines, or extra ones at random spots due to pauses in piping. Personally I feel lack of white space is less of an issue - but I would then make it opt-in and have the default use quite (thanks to you @alxndr :smile: ) or inherit #22 Exhibit A

Do you have any thoughts on this?