Provide command line interface from yasnippet-debug.el

yasnippet-debug.el can now be used to quickly test and debug a snippet
in a file.

* Rakefile (itests): New target, runs tests interactively.
* yasnippet-debug.el: Set lexical binding.
(when-let): Backwards compabtility definition.
(yas-debug-live-indicators, yas-debug-live-colors)
(yas-debug-recently-live-indicators, yas-debug-get-live-indicator)
(yas-debug-live-marker, yas-debug-ov-fom-start, yas-debug-ov-fom-end)
(yas-debug-live-range, yas-debug-with-tracebuf, yas-debug-snippet)
(yas-debug-target-buffer, yas-debug-target-snippets)
(yas-debug-snippets, yas-debug-process-command-line): New functions
and variables.
(yas-debug-test): Remove.
* yasnippet.el (yas--snippet-revive): List snippet identifier in debug
message.
(yas--post-command-handler): Allow the debugger to run.

Rakefile

@@ -20,6 +20,12 @@ task :tests do
     " --batch -f ert-run-tests-batch-and-exit"
 end
 
+desc "run test in interactive mode"
+task :itests do
+  sh "#{$EMACS} -Q -L . -l yasnippet-tests.el" +
+     " --eval \"(call-interactively 'ert)\""
+end
+
 desc "create a release package"
 task :package do
   release_dir = "pkg/yasnippet-#{$version}"