Allow characters prefixed with backslash

Words on the command line in mxsh can be grouped together using single
or double quotes. But these groups can not themselves contains the quote
in question. This is usually allowed by putting a backslash in front of
the character.

Allow this by always including the character after a backslash without
including the backslash itself. If a backslash has to be included then
two backslashes should be used.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>