Add the option to specify a character map when creating a CharSet

With some font bitmaps, the order of the characters isn't always the same.

For example, this is a non CP437 tileset.

The ability to specify the order of characters when creating a new CharSet object would be needed.

So in this case the character map would look like this:

string charMap = " !\"#$%&'()*+,-./" + 
                 "0123456789:;<=>?" +
                 "@ABCDEFGHIJKLMNO" +
                 "PQRSTUVWXYZ[\\]^_" +
                 "`abcdefghijklmno" + 
                 "pqrstuvwxyz{|}~ ";

To specify " or \ characters, escape characters should be used, like so: \" and \\.