Introduce a configurable order of stripping leading components

[mkalkbrenner]

As described in #133 the hardcoded order of p1, p0, p2 is the worst strategy for drupal core patches.
In #136 it is explained that p4 is missing for Magento root patches.
I suggest to turn the hardcoded order of fallbacks into a free configurable one.
Maybe something like
"strip-leading-components": "p2,p1,p0,p4"
in the extras section.

[cweagans]

Definitely not gonna happen in 1.x, but you'll be able to set the patch depth on a per-patch basis in 2.0.0.

[cweagans]

(see #93 for what will be included in 2.0.0)