Batch processing support

• Thanks to @lrw04, batch processing is now supported.
• Recompiled with XCode 13.2.1

Recompiled with XCode 13

Updated project to XCode 13

- Bumped version to 0.4
- Added optimization flag in README.md

Recompiled with XCode 11.1

Also decreased minimal supported Mac OS X version to 10.13.4 according to documentation.

Minor update

• Changed default quality from 0.8 to 0.76
• Recompiled with Swift 5
• Minor documentation improvements

Initial release [Mac OS X Mojave]

Usage: HEIF [-q=quality]
where quality is in range from 0.1 (max compression) to 1.0 (lossless)