Added two required features:
- directives can be commented out using \, as in \$def.
This will treat everything from the \$ up to the end of the line as a comment,
and if the \$ is at the start of the line, will elide the entire line.
This way it is easy to comment out $def DEBUG, for example.
To actually print out the characters "\$", use \$$.
- include now uses a search path, similar to gcc.
Specifying -I<path> adds the path to the search path.
Paths can be relative, in which case they are added to the working directory
from where simtex has been called, or absolute.
Each calling context adds its own directory to the search path, so that
files local to it can also be found. Of course, you can always say
$include path-to/file...
Context paths are seached from top to bottom, ie. newest context first,
so that file inclusion will work as expected.