Add cf customtest command

[user202729]

No description provided.

[xalanq]

Great works!
But I have changed a lot in the local repo. Maybe there would be a lot of conflicts.

[xalanq]

Pushed. You can try to solve the conflicts

[xalanq]

I think language id is not user-friendly. Any thought?

[xalanq]

Maybe you can ask what user wants, just like template configuration.

[user202729]

Comment:

• Docopt doesn't show any error messages when the format of the usage string is wrong. It just silently exits.
• When the user provide an invalid language ID, the program will hang forever.
  It's possible to check whether the ID is valid before submitting, but it will require the language list to be up-to-date (otherwise custom test of newly added language is not possible).

[xalanq]

I think the custom test should be independent of templates. I mean you can pop out an available language list if the template detection failed instead of showing the error message.

[xalanq]

BTW the command customtest could be ctest XD