Mariadb currently supports loading
the datadir. Will add loading
of an sql dump in a coming commit.

Also:
Touching up on the README
Lock all versions in the package.json
file

Test plan:
* Create an empty database with
  sider snapshot add -e mariadb empty.
  Add atleast one database.
* Clone the created empty snapshot.
  sider db start test empty. Make
  some modifications to the snapshot.
  Stop it and start it again. Verify
  changes are persisted over startups.
* Make one more change - then reset
  the database via sider db reset test.
  Start the database and verify
  changes now discarded.
* Make more changes and promote
  the database to a snapshot.
  Clone the snapshot and verify
  changes were transmitted to
  the snapshot.
* Make some more changes, eject
  the database and import it as
  a snapshot. Clone it and verify
  that the changes were persisted
  over the eject and import.
* Verify no regression on existing databases.