enhance custom mode to have allowed characters list and word generation

[jeffalexander]

The two scenarios l have recently had that a custom allowed character list would have helped was:

• to practice my custom symbols layer
• to focus on less letters (such as just the home row when first learning colemak)

A draft idea could be to add a checkbox to the 'custom' mode which would use the input text as an allowed list of characters (rather than currently it is about words). This reduced allowed characters mode would probably also need to generate random words (of random length).

My current workaround is to generate a custom list of words myself and then enable random mode.

Thoughts? Does this sound useful?