Add support for quoting directive names

Now only unquoted, literal names are recognized as directives, for
example:

'print' = abc
print $print

1 files changed, 8 insertions, 0 deletions

diff --git a/tests/directive/buildfile b/tests/directive/buildfile
new file mode 100644
index 0000000..0f83717
--- /dev/null
+++ b/tests/directive/buildfile
@@ -0,0 +1,8 @@
+"print"  = a
+'print' += b
+pr"int" += c
+print'' += d
+
+print $print
+
+./: