feat(mock service): add pactfile_write_mode option handling

[Narazaka]

I want to use the pactfile_write_mode feature for separated test files.

[coveralls]

Coverage increased (+0.05%) to 95.699% when pulling da92274 on Narazaka:master into 5f2c851 on pact-foundation:master.

[mefellows]

This is a great idea!

Have you tested that this gets through to the underlying Ruby process, though (i.e. have you tested this works).

Likely, this would also need to go into http://github.com/pact-foundation/pact-node also, if not into the CLI of the Ruby process.

[Narazaka]

I had tested it with "overwrite" and "update" option values and checked their working fine. The "pactfile_write_mode" option is handled by not cli options but pact mock server's POST /pact API.
The "smart" option seems to depend on rspec so I ignored that.

[mefellows]

Awesome - I'll bring it in, update the docs etc.

This will be useful for a number of use cases!