Add option --custom to perform test with custom servers

[edseldim]

Hello. I added the --custom argument so that users can include a URL like https://www.speedtest.net/api/js/servers?engine=js&search=orange&https_functional=true&limit=1 and perform speedtests against a server of their liking.

Update 7/3/2022:

• added interoperability feature to make --custom work alongside with --server that way we can choose a specific server from custom link.

For instance, a command like speedtest --custom "https://www.speedtest.net/api/js/servers?engine=js&search=Comcast&https_functional=true" --server 37808 should ping Comcast (Atlanta, GA)

[abrar71]

Thank you for this update edseldim, @sivel Please review this pull request its much needed