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.

__tests__/gobin.test.ts

@@ -0,0 +1,21 @@
+import * as gobin from '../src/gobin';
+
+jest.mock('child_process');
+
+describe('gobin', () => {
+  const childProcess = require('child_process');
+
+  let execSpy: jest.SpyInstance;
+
+  beforeEach(() => {
+    execSpy = jest.spyOn(childProcess, 'exec');
+    execSpy.mockImplementation((_command, callback) => {
+      callback('', {stdout: '/home/user/go', stderr: ''});
+    });
+  });
+
+  it('should return ${GOPATH}/bin', async () => {
+    const gobinPath = await gobin.getGOBIN('...');
+    expect(gobinPath).toBe('/home/user/go/bin');
+  });
+});