Removed blurhash dependency

:sparkles: What kind of change does this PR introduce? (Bug fix, feature, docs update...)

This removes the blur_hash OctoSet

:arrow_heading_down: What is the current behavior?

Currently it's depending on blur_hash and blur_hash is broken on current master channel, see https://github.com/fluttercommunity/flutter_blurhash/issues/57 As OctoImage is also used by other packages, such as CachedNetworkImage this also breaks those packages.

:new: What is the new behavior (if this is a feature change)?

It removes the dependency on blur_hash and adds an explanation to the readme how to keep using it. I would say that no included OctoSet should ever add external dependencies to keep this package clean and maintainable.

:boom: Does this PR introduce a breaking change?

Yes, because we are removing an existing API.

:bug: Recommendations for testing

No recommendations, as we only remove some non-critical code.

:memo: Links to relevant issues/docs

https://github.com/Baseflow/octo_image/issues/29

:thinking: Checklist before submitting

[x] All projects build
[x] Follows style guide lines
[x] Relevant documentation was updated
[x] Rebased onto current develop

Files Changed	Coverage
lib/src/errors.dart	`ø`
lib/src/octo_set.dart	`0.00%`
lib/src/placeholders.dart	`ø`

Baseflow / octo_image