Add a -d flag that allows parsing a directory instead of an import path

[dominikh]

The -d flag takes precedence over an import path.

There are two reasons for adding a -d flag

• It makes editor integration easier. Every editor knows the directory
  of a given file, not all editors know the import path
• Being able to parse a directory avoids ambiguity when an import path
  exists in multiple GOPATHs (e.g. when working on someone else's
  package)

[kisielk]

Not sure if I want to have a separate flag for this or not. Doesn't the gotool or godoc handle this automatically?

[dominikh]

godoc seems to handle absolute paths. If you want me to, I can adjust the PR to differentiate between import paths and absolute paths and drop the -d flag.

[dominikh]

Hm, I am not too sure how godoc behaves. /foo/bar is considered a file path, foo/bar is not, but ./foo/bar is again.

https://code.google.com/p/go/issues/detail?id=4330 seems to clarify the behaviour.

[dominikh]

I changed the behaviour to be more like godoc. I rebased the feature branch to have a clean single commit. No more -d flag.

[kisielk]

I'll make the changes myself later.