Add support for Laravel 5.6

Updating Symfony version constraints

#163

Don’t build Flysystem disks until needed

Instantiating those disks actually created directories in the filesystem.  This caused problems with `php artisan storage:link` where Croppa would create an empty public/storage immediately before the symlink was created.  It was a side effect of the storage class getting booted because it was a dependecy of many classes that get insantiated with artisan.

Laravel 5.5 support, pad method

• Supports Laravel 5.5 and adds package auto-discovery
• Add's option to pad images with a color #140

Fixes issue on first image response

See #134

Custom filters and manual rendering

• Adding custom image filtering options (#123)
• Adding Croppa::render() method to render crops directly (#111, #124, #125)

Memory limit config & Laravel 5.2 support

• Allow the required memory to be set (#116)
• Support Laravel 5.2 via opening up Symfony version constraints (#119)

Lumen support & fixes

• Adding Lumen support to the provider
• Fixing issue with deleting crops NOT in a subdir #103
• Refactoring Flysystem caching adapter support #87

Fixing signing_key reference

Merge pull request #100 from sdebacker/patch-2

signing_key, not secure_key.

Adding signed tokens to generated URLs

Now, by default, urls generated with Croppa::url() will have a ?token=xxxxxxxx parameter added to them. The token is generated by hashing the parameters of your request (src image, dimensions, etc) combined with your Laravel app's encryption key. For example, a new Croppa 4.1 URL might look like:

http://domain.com/uploads/01/08/file-200x100.jpg?token=c21ed23af0228053c10b283a93b30d8c

This prevents the modifying of Croppa parameters after url generation which could lead to malicious users consuming CPU and storage generating unnecessary crops. As such, it more effectively accomplishes what the max_crops config was designed for; max_crops is now disabled by default.

If you are generating URLs outside of Croppa::url(), like the croppa.js module, you can disable this feature by setting the signing_key config to false.

Thanks to Glide for the inspiration for this feature.

Adding a command that deletes all crops

Run php artisan croppa:purge from the CLI to delete all crops. See the README for more info.