Add local cache for WordPress core download

Cache `wp core download` files after the first run. To speed up the process of installation and reduce unnecessary downloads.

When cache is empty: (These should be skipped if user has passed (`--version` flag while installation)):

1. For first install run `wp core download`.
2. Cache the downloaded WordPress file in EasyEngine cache directory (`EE_CACHE_DIR`).

On next install:

1. Copy the cached files to htdocs.
2. Run `wp core verify-checksum --version=$(wp core version)` (Need better way to verify integrity of cache files. `wp core version` can be maniuplated if the files are wrong). On failure purge cache and proceed with cache empty scenario.
3. Once checksum is verified. Run `wp core update` to check for update and update to latest version of any.
4. If WordPress gets updated in the above, cache the updated files.