allow tmp dir customization via env variable

This is the old story of Appium moving an app to the tmp dir and then crashing. The tmpdir is cleaned or pc restarted and we loose the app.

Loosing builtin apps can be prevented by configuring APPIUM_TMP_DIR in shell startup script.

@jlipps is it ok? ping @Jonahss.
This is wrong for Windows and will cause a lot of problems.  Better to do this:
```js
this.args.tmpDir = process.env.APPIUM_TMP_DIR ||
	(helpers.isWindows() ? process.env.TEMP : "/tmp");
```