Option to pull assets from a CDN

[adamgruber]

Currently mochawesome supports inline and external assets. External assets are copied into an assets/ folder in the directory where the report is generated. Copying these assets adds overhead. In the case where generating multiple reports into different locations, this overhead may be considered undesirable. Inlining the assets would solve the issue but also bloat the report filesize.

If the assets were requested via CDN like unpkg.com then the overhead would be eliminated. I propose adding this as an option.

Obvious note: Serving the assets via CDN requires an internet connection when viewing the report.

[adamgruber]

Available in 3.1.0