add directory argument for dep_usage_proj, #11

[jsta]

Avoids throwing the error described in #11 and adds a directory argument to dep_usage_proj.

[jnolis]

I was having this same error from #11 and this fixed my issue. Thanks for making that change! However, for dep_usage_proj shouldn't the defaults then be function(path = ".", dirs = ".")? It's hard for me to imagine a situation where an R user would have created an R project and also would have exactly mimicked a package layout while having not created a package.

[jsta]

Your suggestion makes sense to me. The downside I see is that it would not be backwards compatible with previous code.