new feature: add the option --wordchars, to set extra word characters

This allows the user to specify which other characters, besides the
default alphanumeric ones, should be considered as part of a word, so
that word operations like Ctrl+Left and Ctrl+Right will pass them by.

Using this option overrides the option --wordbounds.

This fulfills https://savannah.gnu.org/bugs/?47283.
10 files changed