Work with latest crystal version

Updated SourceCustom and TargetCustom to work with latest Crystal and libvips version.

v0.1.5

some refactoring and adding new enum member. Includes PRs #5 and #7 . Thanks @Sija for PRs

Fix memory usage issue in Image

v0.1.4

fixed memory usage issue

Fix memory usage issue

Fix issue #4

Updated to LibVips version v8.14.2

v0.1.2

Updated to LibVips version v8.14.2

Release for Crystal 1.5.0

v0.1.1

Updating version to 0.1.1

Initial release

0.1.0

Initial commit