Add support for quoting directive names

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

'print' = abc
print $print

1 files changed, 3 insertions, 0 deletions

diff --git a/tests/directive/test.sh b/tests/directive/test.sh
new file mode 100755
index 0000000..b898b3c
--- /dev/null
+++ b/tests/directive/test.sh
@@ -0,0 +1,3 @@
+#!/bin/sh
+
+valgrind -q b -q | diff -u test.out -