support different working directory (#64)

* support different working directory

* validate working_directory

__tests__/workdir.test.ts

@@ -0,0 +1,11 @@
+import * as workdir from '../src/workdir'
+
+describe('isValid', () => {
+  test('throws an error if working_directory starts with /', () => {
+    expect(workdir.isValid('/usr/local/bin')).toBeFalsy()
+  })
+
+  test('throws an error if working_directory starts with ..', () => {
+    expect(workdir.isValid('../../etc')).toBeFalsy()
+  })
+})