Add Checkbox and Radio Group fields to Micron

[RFnexus]

This PR adds two new input field types to the Micron markdown format. The two fields use either a ^ (radio) or ? (checkbox) after the < in the field markup.

Checkboxes

Checkboxes allow users to make a single selection or multiple selections. They are rendered using the Urwid CheckBox widget.

Example:

`<?|field_name|value`>`b Label Text`

• field_name: The name of the field. Multiple checkboxes can share the same field_name
• value: The value submitted when the checkbox is checked. If their are multiple checkboxes with the same field name, but different values it will concatenate all checked values with a comma.

Radio select

Radio selects allow users to choose one option from a list. They are rendered using the Urwid RadioButton widget.

Unlike checkboxes radio buttons with the same field name are mutually exclusive.

Example:

`B900`<^|color|Red`>`b  Red

`B090`<^|color|Green`>`b Green

`B009`<^|color|Blue`>`b Blue

In this example, when the data is submitted field_color will be set to whichever value from the list was selected.

Both checkboxes and radio groups can be pre-checked by appending a |* after the field value

Additionally, for LXMF clients like Liam Cottle's MeshChat that are browser-based, these can be mirrored using the native HTML radio select and checkboxes.

[markqvist]

Great work! Thanks so much for this PR! If you feel it's ready for it, I'll merge it now for upcoming releae.

[RFnexus]

Great work! Thanks so much for this PR! If you feel it's ready for it, I'll merge it now for upcoming releae.

Should be good to go

[markqvist]

Very nice! Merged!