Add custom conf dir option and fallback shipfile location to current working directory

[joshrowley]

Okay, so I think the majority of the time, your ship.conf file isn't going to be located inside of the directory you are deploying since you wouldn't want to expose the contents of that file. However, right now ship only looks for the conf file inside of the folder being deployed.

This PR adds the following behavior:

• If a user gives a conf option (-c, --conf on the command line), ship uses that location to set the shipfile path
• If there's no shipfile inside root (the folder being deployed), ship looks for a shipfile in the current working directory.
• If there's no shipfile inside root nor cwd, it sets the shipfile to be inside root so that the config prompt will write it to the correct location. My opinion is that this should be process.cwd() as well, I think it's dangerous to write the conf file into the folder that's being deployed, but I'd like to hear anyone else's thoughts on this before I make the change.

[jescalan]

Hmm yeah I agree on the third point that it should write into cwd. Most deployers refuse to push a shipfile by auto-adding it to ignores so I think it should be safe.

[joshrowley]

Cool, I updated it to write to cwd. I didn't realize the shipfile ignore was set on a per deployer basis, would it be a good idea to set this globally like this, instead of within each deployer?

[jescalan]

@joshrowley yeah that seems like a good fix!

[joshrowley]

@jenius Does this PR look good for a merge? The only failing test is the broken heroku deployer.

[jescalan]

Yup!