Implement optional module loading

The syntax is:

using? cli

Now each module use results in two bool variables: <module>.loaded and
<module>.configured.

Also implement variable visibility (the above two variables are limited
to project).

1 files changed, 5 insertions, 0 deletions

diff --git a/tests/using/buildfile b/tests/using/buildfile
new file mode 100644
index 0000000..86baa42
--- /dev/null
+++ b/tests/using/buildfile
@@ -0,0 +1,5 @@
+using? foo
+print $foo.loaded
+print $foo.configured
+
+./: