Enable “multisheet” support when creating new Ethercalcs.
Enable “multisheet” support when creating new Ethercalcs.

Prepend `=` to the Ethercalc name when creating new calcs so the
multisheet support is enabled.

More information:
  — https://github.com/audreyt/ethercalc/issues/138
  — https://github.com/otetard/ownpad/issues/26

Closes: #26

otetard/ownpad,otetard/ownpad