Print character only if printable

[Hywan]

Fix #16 and #17.

When a character is not printable, the readline did still print it. In some cases, it could create weird bug (see 1 or 2). Since we have the Hoa\Ustring::isCharPrintable, we can check whether a character is printable or not. Consequently, here is the new logic:

• We read a character,
• The buffer receives this character,
• If there is no mapping for this character and if it is not printable, then we do not print it. We do nothing with it.

So, if we would like to support a control character that is not printable, we must associate a mapping to this character (like we do for Ctrl-A for instance).

[Hywan]

Previous commit avoids to print a non-printable character. This one just add a “bell”/“bip” when the user did press such a character. This is a sonor indication that something wrong happened.

[Hywan]

Asking a review from @hoaproject/hoackers :-).

[vonglasow]

:+1: