Loading default parameters value in client

[ekorian]

Add an ability to load a file with default parameters value within the client. This is useful in the case of a capability with multiple parameters whose defaults value are used most of the time, to avoid having to set them every single time.

[britram]

A generalization of this: make it possible to (1) dump and load capabilities as JSON envelopes and (2) load specifications as JSON in mpcli. Provisionally assigned to Martin Müller (MSc student).

[muemart]

Implemented in my fork. Though instead of dumping capabilities, it dumps them as specifications, because it's either enforced that capabilities don't have parameter values, or they get interpreted as constraints when importing. This requires that parameters are set inside mpcli when exporting, which is a bit annoying.
You can dump (multiple) specifications to a file, load parameters, or directly run a specification. Did I miss anything?