Add array selector support

[mikee47]

Implement array selectors as discussed in #18.

Also update import methods to return Status which explains source of any error.

TODO:

• [x] Review test cases
• [x] Review documentation
• [x] Array tests are sluggish on esp8266, find out why. Partly because of saving store after each run. Add clearDirty method to prevent this.
• [x] Review error handling