Load setup files instead of sourcing them

- needed for same setup files to work on Windows
  (where PowerShell is used instead of bash)
- removes ugly bashism to allow overriding from .travis.yml
- adds a proper (yet small) syntax
Also adds tests for the new syntax and updates the README.

test02.set

@@ -0,0 +1,10 @@
+# a comment, then an empty line
+
+  # a comment that is indented
+BASE=foo
+# include an existing file
+include test01
+
+FOO=bar
+FOO2=bar bar2
+  FOO3=bar bar2