Support arbitrary blueprint sources

[davewasmer]

(moved from denali-js/core#59)

When I run $ denali generate, I should be able to supply:

• A local directory, i.e. $ denali generate ../some/local/dir
• A git url, i.e. $ denali generate https://bitbucket.org/user/some-cool-blueprint
• A github repo? i.e. $ denali generate user/some-cool-blueprint