Add option for specifying external dependencies

[chshersh]

Can be helpful for creating more customizable scaffolds. The TOML can look like this:

dependencies =
    [ { stanza = "library", dependency = "tomland ^>= 1.3" }
    , { stanza = "test", dependency = "hspec ^>= 2.7" }
    , { stanza = "test", dependency = "hedgehog ^>= 1.0" }
    ]