Implement reading in ASCII art file

[szimmer9]

Different colors in the ASCII art image is denoted by differing symbols. Each symbol specified (ie @, #, $, %, ^, A, a, 1, etc) can correspond to a different color in the header. Otherwise, all the pixels will be set to an arbitrary color. The end of the header is denoted with a tilde (~). Spaces are reserved for the background and tildes are reserved for the end of the header, so these cannot be used for symbols in the image.

Header information will have the following format:

width height
symbol red_value green_value blue_value
~
<image>

Example:

15 13
@ 255 0 0
* 0 0 0 
# 255 255 255
~
<image>

Additionally, this will require updating the convert script to allow specification of the a option.