support pre-filled databases

[kurtdevos]

This allows the same plugin wrapper to be used with an alternative version that supports pre-filled sqlite databases.
See https://github.com/litehelpers/cordova-sqlite-ext

Installing sqlite-ext:

ionic cordova plugin add cordova-sqlite-ext --save
npm install --save @ionic-native/sqlite

Opening an existing database requires the "createFromLocation" param:

this.sqlite.create({
      name: dbName,
      location: 'default',
      createFromLocation: 1
    })

This patch adds that param as an optional parameter.