Add units to Gen input

[nai888]

Is your feature request related to a problem? Please describe.
Gen currently allows options but no other features. Adding units (options surrounded by [ ] brackets) will improve succinctness of input.

Describe the solution you'd like
Add the ability to use [ ] brackets to demarcate single units. For example, the code s[tu/top/kan] will produce the output stu stop skan.

Additional context
You can find the instructions for using this feature within Awkwords in its help file.

Related issues
#63