fix: Introduce cross-spawn for cross platform commands (#175)

This PR introduces cross platform spawning of commands. Will close #174. 

The problem is Windows users need to use the `exec` command like this: 
```
$ percy exec -- cypress.cmd run
``` 

If they don't, we can't spawn their original test command. What's happening is nodes `spawn` ignores windows `PATHEXT` which is kinda like `$PATH` & that means `spawn` only works on Windows for `.exe` files. See: 
- nodejs/node-v0.x-archive#2318
- bcoe/awesome-cross-platform-nodejs#26

Tested here: https://github.com/percy/example-percy-cypress/compare/rd/test-cross-platform

The first commit uses `cross_spawn` and successfully starts Cypress. The second commit uses `0.4.0` of agent and fails to start Cypress.

package.json

@@ -29,6 +29,7 @@
     "body-parser": "^1.18.3",
     "colors": "^1.3.2",
     "cors": "^2.8.4",
+    "cross-spawn": "^6.0.5",
     "express": "^4.16.3",
     "globby": "^9.2.0",
     "js-yaml": "^3.13.1",
@@ -52,6 +53,7 @@
     "@types/chai": "^4.1.4",
     "@types/chai-http": "^3.0.5",
     "@types/cors": "^2.8.4",
+    "@types/cross-spawn": "^6.0.0",
     "@types/http-server": "^0.10.0",
     "@types/mocha": "^5.2.5",
     "@types/nock": "^9.3.0",

src/commands/exec.ts

@@ -1,5 +1,5 @@
 import {flags} from '@oclif/command'
-import {spawn} from 'child_process'
+import * as spawn from 'cross-spawn'
 import Constants from '../services/constants'
 import PercyCommand from './percy-command'