Windows support

[pauljz]

It'd be great to have Windows support in srclib for cross-platform editors.

Go itself supports Windows just fine, so most of the fixes that need to take place for this are oriented around taking advantage of Go's standard library. Most of the fixes are really simple things:

• Using filepath.SplitList(paths) instead of strings.Split(paths, ":")
• Using os.PathSeparator and os.PathListSeparator instead of ":" and "/"
• Using os.TempDir() to get temp paths instead of hardcoded /tmp/ paths.

There are some more complex issues:

• srclib relies on sh -c for executing certain commands.
• srclib uses symlinks heavily, which while they work on Windows aren't terribly idiomatic. And by default cannot be created by a non-elevated process. (e.g. equivalent of having to sudo to symlink, ugh). We may need to provide a fallback that uses directory copies.
• Some of srclibs dependencies use different formats for command line flags /methods:program instead of --methods=program.
• And then of course there's Docker, which Windows at least now has some nascent support for. It's not clear to me how much will need to change to support this.

I've started on some fixes for a lot of these issues and will put a PR together soon once I'm sure the tests pass. I'll probably need a lot of help from folks that are more experienced with the srclib ecosystem, since this is all new to me.

[dmitshur]

Hey @pauljz,

Windows support sounds great! The easy parts like using filepath.SplitList should definitely be done regardless, since it's more correct. I've recently made a PR https://github.com/sourcegraph/srclib-go/pull/35 to fix some of those. There are likely more things like that remaining.

I don't have a Windows machine to test on, but I can try to assist in other ways. I'll keep an eye on this and help review your future PRs.

[pauljz]

This round of changes should be ready to review now, @shurcooL. I took out the changes to toolchain_cmd that were breaking the build, changed all refs to os.PathSeparator to filepath.Separator for consistency, and changed erroneous path.Joins to filepath.Joins.

Travis build passes, all tests pass locally now on OS X for me. Windows tests... well, still a long way to go there, but it's better than it was.