Add a MONGOOSE_NO_URL_DECODE flag

For certain applications, it is essential that
http://<host>:<port>/path/with/inline%2fslash/
and
http://<host>:<port>/path/with/inline/slash/
are different.
Currently, mongoose always decodes the URL from the HTTP header,
thus irreversibly loosing that distinction.
This flag gives the user a chance to change this behaviour.

mongoose.c:
    Do not decode the request uri path if MONGOOSE_NO_URL_DECODE
    is defined.

docs/Embed.md:
    Document the MONGOOSE_NO_URL_DECODE flag.