Added option to use variable $project_path in any part of config when it related to running exteral toolkit.
It gives option to use project-related settings relative to project root. For example we can specify path
for phpmd ruleset xml "args": [ "$project_path/phpmd-ruleset.xml" ]. In that case also settings will be linked
to project root and sublime project file will not contain full paths, so project will work fine from any location.