Add makefile variables and default rule

Also turns out I forgot to add tree2dot.c as a dependency,  so that was
fixed
Skip whitespace at beginning of input

Fixes a parse/crash issue where the whitespace at the start of input
would try to be parsed as a node value string but the tree wasn't made
yet.
Added support for C99-style one-line comments

Comments are treated like whitespace and are skipped over.
Added comment to test.txt example