cached 0.5.0

# Changelog

## [0.5.0]
## Added
- Add `cached_key` macro to allow defining the caching key

## Changed
- Tweak `cached` macro syntax
- Update readme

## Removed


## [0.4.4]
## Added

## Changed
- Update trait docs

## Removed


## [0.4.3]
## Added

## Changed
- Update readme
- Update examples
- Update crate documentation and examples

## Removed