Add a "watch_patterns" (opposite of ignore_patterns)

[tomcornall]

Suggested by a customer - like using "ignore_patters", a "watch_patterns" could denote patterns that are being watched by LemonSync, and only those denoted are being watched.

By adding this, we could include a smart set of default "watch_patterns" that followed the default theme structure that we expect, for instance:

{
  "store": "http://testetattaet.local.io",
  "theme_code": "zest",
  "api_token": "1244j11kj2hf34hj12g4",
  "ignore_patterns": [
    "*.tmp",
    "*/.git*",
    ".ds_store"
  ],
  "watch_patterns": [
    "content/",
    "pages/",
    "partials/",
    "resources/",
    "templates/",
    "package.json/",
    "theme.yaml/"
  ]

This could lead to many fewer unexpected theme stuff being uploaded by mistake - think large node_modules folders.