#35 Added docs/DESIGN, tried fixing variable expansion.

Variable expansion works when the specified variable exists, but if the
variable does not exist then the command would get in an infinite loop
because it would not change the command and would re-dispatch the
command, bringing the command right back to the variable expansion
interceptor.

To fix this, the variable expansion interceptor now escapes the variable
markers of variables that do not exist, and now skips variables that are
escaped.

However, it doesn't work completely: For instance, running
`echo $POP` should either echo the value of $POP if it exists, or should
echo "$POP" if it does not exist. However, "$POP" is not being echoed.
I'm not sure why.