Add $GOBIN

- Set $GOBIN to $(go env GOPATH)/bin
- Add $GOBIN to the PATH

Should make the setup of tools like golangci-lint or golint work with a
simple `go get`.

Using $GOBIN instead of $GOPATH/bin because the goal is to have GOPATH
not being directly referenced. Also, in the future, GOBIN will have a
default value too, so we would not need to manually set it, just add it
to the path (see discussion in golang/go#23439).

Closes #14.

src/gobin.ts

@@ -0,0 +1,13 @@
+import * as childProcess from 'child_process';
+import * as path from 'path';
+import {promisify} from 'util';
+
+const exec = promisify(childProcess.exec);
+
+export async function getGOBIN(installDir: string): Promise<string> {
+  const goExecutable = path.join(installDir, 'bin', 'go');
+
+  const result = await exec(`${goExecutable} env GOPATH`);
+  const gopath = result.stdout;
+  return path.join(gopath, 'bin');
+}