Flag to skip parsing on PE object instantiation

[0xdevalias]

When creating a new instance of PE, the current processing immediately tries to parse the data. When trying to analyse a corrupted file piece by piece, this makes things much harder than they probably need to be.

I'm proposing a new parameter to PE's __init__ (skip_parse?), and a new check around the call to self.__parse__ on line 1713

If you're happy for this, I can write up a quick PR.

Relates to #124