updgrade csscolorparser to v0.1.5

benchmarks

bring back Clamped()

Add donate button

Delete unused images

change behaviour of GradientBuilder's .colors() and .html_colors()

Add .codecov.yml

Refactor

- Remove SharpGradient(), IntoColors()
- Add new methods for Gradient:
    + ColorfulColors()
    + Colors() -- now it returns []color.Color instead of []colorful.Color
    + Domain()
    + Sharp()
- Refactoring

Named colors

+ Rename gradient builder's HexColors() to HtmlColors()
+ Add support for named colors

Patch4 (#18)

* Add files via upload

* Update README.md

* Add files via upload

* Update README.md

* Update gradx_test.go

* Update gradx.go

* Update gradset.go

* Update gradset_test.go

* Update gradset_test.go