Allow passing custom CSS file to html_coverage

[jagot]

I added a keyword argument to html_coverage that takes the path to a custom CSS file, to be used as

html_coverage(cov, css="path/to/custom.css")

Using e.g. a CSS file based on the Gruvbox theme, it can look like

gruvbox.zip

[tpapp]

Thanks, just requested a tiny change, and please add a test. It can be something trivial, eg

1. you provide a CSS file and then grep for it in the result, and

2. you provide a bogus argument (eg an integer) and it errors.

[tpapp]

Suggested change

	function html_coverage(coverage::PackageCoverage; gitroot = ".", open = false, dir = tempdir(), css::Union{Nothing,String}=nothing)
	function html_coverage(coverage::PackageCoverage; gitroot = ".", open = false, dir = tempdir(), css::Union{Nothing,AbstractString}=nothing)

[jagot]

Thanks for the suggestions!

I added a test for the argument error you suggested, and generation of a coverage report using a dummy CSS file, testing for equality using the FileCmp package as a extra test dependency. Hope that is OK.

[tpapp]

Thanks for the very clean code.