Add variable code mode and some features and configurations

[shuGH]

It's a very wonderful extension, and thank you for a release. I tried to add some new features and updatings. If there is nothing wrong and you like, please merge it!

• Add character config used to generate code.
  • Character order is correspond with priority to use.
  • Fixed #20 (e.g. set "1234567890")
• Change target range to current visible.
  • Fixed #29
• Add incremental code display.
  • Filtering code each time you type.
• Add variable code mode. You can select this mode in a configuration.
  • Fixed: As before. And you can set code length.
  • Variable: According to the target number, code length is variable.

Fixed

Variable

Thanks ( • ̀ω•́ )و☆

[wenfangdu]

Most of the mentioned features have been implemented in Jump.