Allow paths to be defined using variable references in RHS of assignments

[dowobeha]

It would be nice to allow the following:

task foo :: dir=path/to/dir

a=$dir/a.txt b=$dir/b.txt {

... Bash code ...

}

[jhclark]

If you can capture variables that are part of paths, I can easily dereference and do substitution in DirectoryArchitect.scala

[jhclark]

Note: This implies that param variables will take precedence over global variables within the scope of a task when using the $var syntax.