Expand SolidHost config constructor, plus tests

[dmitrizagidulin]

Minor refactor, expand the SolidHost constructor.

Extracted from PR #702, partly addresses issue #483.

[kjetilk]

Looks good to me, but I don't quite understand the impact of just passing argv, so I think there should be more reviews.

[dmitrizagidulin]

@kjetilk so, the impact of the change (just passing in the argv config object directly) is quite minimal. You can think of the SolidHost instance as a class that holds (some of) the config attributes of argv.

[michielbdejong]

LGTM, assuming that there will then be other code that reads these new variables (host.live, host.root, host.multiuser, host.webid). Is this adding them as runtime configurable (as opposed to read from the config)? If so, then they should also be added to the usage documentation somewhere, probably?

[dmitrizagidulin]

@michielbdejong Yes, there is downstream code that checks those params. (And no, they're not live-configurable, they're just from the config file only)