Export Media Library

• Bumped minimum PHP version to 7.4

• adjust syntax to ensure plugin compatibility check can run on older php versions (eg. PHP 5.2)
• removed dependency on ext-mbstring by allowing mbstring polyfill to be used as fallback
• export zip filename now incorporates blogname and utc date for better consistency and organization
• clean and end all output buffers by default to ensure PHP doesn't store zip archive in output buffer and run out of memory

• bumped minimum php version to 7.1
• flush buffer after every write in order to avoid exceeding memory

• added support for PHP 7.0

• expose API::export function for easier reuse by 3rd-party code
• set last modify time for each file in zip to match the timestamp on disk

Fixed title of plugin in readme.

• Fully functional release.