Allow watching multiple paths for changes

[rselbach]

Hi, this change allows gin to watch multiple paths for changes.

The PR also adds a new parameter (--cwd or -d) to specify where to build from. It defaults to the current directory which is how it worked before the change.

The parameter --path now can include multiple paths in the format of that PATH variable (i.e. a list of paths separated by os.PathListSeparator)

Please consider merging. Thanks, -rst.

[tdterry]

+1.

We build shared libraries for some of our projects. I would love to run gin in two application folders, each of which watches the local folder and the shared folder. Then when I make a change in the shared project, both applications rebuild.

[alvarowolfx]

+1

[ghost]

+1

[m1ch0pv]

+1

[ibash]

Sorry for the delay. It looks like there's conflicts with this, if you want to resolve them I'd be happy to merge.