Feature/deletes #215

[bplatz]

This addresses #215 and adds a deletion capability to Fluree which can leverage a query :where clause to define exact data to remove. This is a substantial increase in capability to current Fluree's delete capability which always requires one to know the subject being removed, and anything more complex than simple deletes can require compare and swap operations.

You can now issue the following as examples (see new tests for full detail)

Delete everything for a subject:

@(fluree/stage db
{:delete [:ex/alice '?p '?o]
 :where  [[:ex/alice '?p '?o]]})

Delete all values for a subject+predicate:

{:delete [:ex/bob :schema/age '?o]
 :where  [[:ex/bob :schema/age '?o]]}

What's completely new however is that you can target other information based on predicates + values:

Delete all subjects that have an email address:

{:delete ['?s '?p '?o]
 :where  [['?s :schema/email '?x]
          ['?s '?p '?o]]}

Delete all subjects who are 30 years old:

{:delete ['?s '?p '?o]
 :where  [['?s :schema/age 30]
          ['?s '?p '?o]]}

[cap10morgan]

Looks great overall! A few questions / little things to address, but good to merge after that.

[cap10morgan]

Is group-by applicable to deleting?

[cap10morgan]

s/Jane and Cam/Jane and Bob/ ?

[cap10morgan]

Should this be something like "Bob should no longer have an age."?