Contracts should include a local copy of their template

[shonfeder]

This is motivated by two intersecting dynamics:

1. The person internal to an organization should have access to all contract templates and their entire history, but external parties signing a contract should not. So we cannot rely on all parties being able pull the template from the source repo.
2. We will almost certainly need to support ad-hoc negotiations and changes (see #23) and such changes need insight into the template, but shouldn't be shared with all parties (even internal parties).

Towards implementation: The prepare subcommand proposed in #46 could be used to prepare a directory that includes

• a copy of the template, along with a url+git ref to its source in space-time and a hash verify it's original contents
• a partially filled config, ready for the other party to complete

[shonfeder]

Labeled as priority as this is requisite to satisfy the following major workflows:

3. Edit contract lines and keep the edits recorded as a diff from the template in an accessible manner.
4. Counterparty can easily comment on the contract or ask questions and Internal Ops to be able to easily respond.

[thanethomson]

Implemented as discussed on #23 (currently on branch prototype/v2, soon to be merged to master).