relax URL encoding

[benoitc]

url path encoding is very strict for now and partially based on the code found in chromium:
https://github.com/benoitc/hackney/blob/master/src/http/hackney_url.erl#L367

A lot of server don't decode all the URL like they should which trigger the issues #246, #272.

New version of the code will allows people to use their own encoding schema by providing the following options:

• possibility to tell to hackney to not encode by itself the URL (raw option)
• possibility to exclude some chars in the path