outputPath should default to tmp/deploy-dist or be configurable

[jaydgruber]

ember-cli-deploy-build defaults to tmp/deploy-dist for the build output, and many of the plugins developed since assume this.

In this repo it is hardcoded to ./dist. This should either match the default above or be configurable.

ember-cli-deploy-s3#distdir leads to me to believe that the outputDir should be passed along in the context object, but I'm not sure I understand all of that yet

[jaydgruber]

I think based on https://github.com/deployjs/webpack-build/blob/master/index.js#L54 that this does get passed along in the context object, so I think downstream deploy plugins should pick up dist/ as the directory to look in just fine. So this change isn't needed for things to work I don't think, just for consistency with conventions in other ember-cli-deploy plugins