Be smart about the main attribute on package.json

[giggio]

If napa detects that the downloaded files don't contain a package.json, it currently creates one. I would add that it should be smart about the main attribute.
If there is a file on the root with the same name of the package, use that as main. For example, if I specify that I want to use jquery-validation-unobtrusive, and that the name I want to use for it is jquery.validate.unobtrusive, it should set the main to jquery.validate.unobtrusive.js, which is a file of the same name that is located on the root.
Maybe also check /lib and /bin.
This is somewhat connected to what was proposed on #18, but it would offer a convention, so I don't always need to configure.

[shama]

+1 Sounds good. Do you know of a good package on npm for detecting a potential main filename?

[giggio]

@shama No, I don't. :( But it should not be difficult to implement this logic I suggested.

[sparty02]

Related: If there is a bower.json in the package root, it would be convenient to pickup the main from that too!