Allow passing H1 Credentials as variables

[rzhade3]

@rzhade3:

Currently, we only support instantiation of the H1 credentials as Environment variables. This means that it is very difficult to use in an environment with multiple programs.

In order to fix this, we should allow instantiation of the H1-client with the creds passed in as variables, and allow callers to decide where the credentials originate from.

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Transferred from oreoshake/hackerone-client#67