Handling custom runes mapping

[goten4]

In GSM7 official table, the character ç does not exist.
It would be great if it was possible to customize charset tables.

In my case, I could map ç to 0x09, to obtain something like this : hameÇon, instead of getting an error.

What do you think ?
Of course I can make a PR if you like !

[goten4]

Another thing about charset tables, it would be great if we can have the possibility to set a default character in case of a mapping is not found (instead of returning an error).
If the default character is not set, an error is returned, and if it's set, the encoder can use it.

[bachhh]

Good suggestion, I'm working on a PR for this.