Allow setting a custom Carpentry type

[milanmlft]

Allow setting a custom value for the carpentry field in the config.yaml of a lesson.

At the moment, if a user wants to create a course website using the Carpentries workbench but with custom styling and logos, they would have to clone both the {sandpaper} and {varnish} repos:

• {varnish} to modify the styling and add custom logos
• {sandpaper} to modify the which_caprentry() and which_icon_carpentry() functions so that it can pick up the alternative logos from varnish.

See for example https://github.com/HealthBioscienceIDEAS/microscopy-novice, which uses the HealthBioscienceIDEAS/sandpaper and HealthBioscienceIDEAS/varnish forks.

This is rather cumbersome, as one needs to clone the entire {sandpaper} repo and then keep maintained, just to modify two lines of code.

With this PR, a user can use any value in the carpentry: config field and it will be picked up by {sandpaper} and used to get the matching logo from {varnish}, as long as there are logo files present in the {varnish} fork being used that (exactly) match the carpentry name. So this omits the need to fork {sandpaper}, only {varnish} needs to be forked.

[tobyhodges]

This looks good to me, thank you @milanmlft. @froggleston @ErinBecker do you think we should add a test to ensure that the newly-defined default is being captured?