Support cut numbers

[qsantos]

https://en.wikipedia.org/wiki/Morse_code#Cut_numbers

[qsantos]

All cut numbers correspond to a character (see table below). So, for decoding, no information is lost by just using this mapping. For encoding, there is too much complexity for little gain in implementing a way for the user to ask for cut numbers. The simplest way is for them to write the normal characters directly.

Number	Long Form	Cut form	Normal mapping
1	.----	.-	A
2	..---	..-	U
3	...--	...-	V
4	....-	....-	4
5	.....	.	E
6	-....	-....	6
7	--...	-...	B
8	---..	-..	D
9	----.	-.	N
0	-----	-	T

However, it would be useful to have mappings from numbers to letters and back.