Accept stdin if no non-flag args are provided

Makes it more usable in scripts, makefiles etc.

Also `go fmt` the whole tree. Could have saved a confusing diff if that
have been used in the first place.

----

Example of the same behavior: https://code.google.com/p/go/source/browse/src/cmd/vet/main.go#91

PS: Please always use `go fmt` before publishing any code. Not `go fmt <anyone's-personal-preferences-options>`, just `go fmt`. Instead, please set your preferences in your favorite editor, to show the code in the way you prefer.

Note: In case you'll accept this pull request then you have to s/cznic/matm/ at https://github.com/cznic/gocov-html/blob/master/gocov-html.go#L25