Improve dev experience with schema-tx literal

[mtnygard]

Several minor improvements and one medium-sized change.

• Early, friendly assertions if inputs are not valid.
• Docstring is now required and won't accidentally eat the lost option toggle. (See #44).
• Multiple option toggles can be added in one row.
• New option toggle :no-history corresponds to Datomic's :db/noHistory true

Fixes #44

[halgari]

A few thoughts to consider on this:

• Vectors tend to imply ordering, schema attribute definitions should order independent
• Currently we assume that the first thing is the name and the rest of the things are cardinality and kind, etc. This is an implied order the programmer has to remember
• Multiple nested vectors are hard to mentally parse be some programmers (like myself).

Suggested change. What if we just restructure the DSL a bit:

#vase.schema-tx {:person/name #{:one :string :unique "A Unique name for the user"}
                 :address/zip #{:one :long "A zip code for the user}}