Split the cache CLI command into clear and inspect

[Abbe98]

Currently the top-level cache command is used both for inspecting a cached item and invalidating it:

Invalidate/clear:
snowman cache countries.rq --invalidate

Inspect:
snowman cache countries.rq

This should instead be split so that it reads better and is easier to document:

snowman cache clear countries.rq

snowman cache inspect countries.rq

[Abbe98]

More non-obvious changes related to this and #91:

snowman cache clear synonym to snowman cache inspect summarizing the cache from both SPARQL queries and remote resources(#91). Not very obvious but it might be dangerous to make it just clear everything.

snowman cache clear --unused-sparql
snowman cache clear --unused-resources (#91)
snowman cache inspect --unused-sparql
snowman cache inspect --unused-resources (#91)

snowman cache clear --all-sparql
snowman cache clear --all-resources (#91)
snowman cache inspect --all-sparql
snowman cache inspect --all-resources (#91)

snowman cache clear --all
snowman cache inspect --all (same as snowman cache inspect)