Implement a custom format parser

This commit is a major change to ramble's expander logic. Previously, we
relied heavily on Python's f-string format syntax. This causes issues
related to formatting template files (like json and yaml), and prevents
some usecases (such as in-line variable definitions).

This change adds a custom parser for processing strings while they are
being expanded.

lib/ramble/ramble/application.py

@@ -921,6 +921,7 @@ def _make_experiments(self, workspace):
 
         for template_name, template_conf in workspace.all_templates():
             expand_path = os.path.join(experiment_run_dir, template_name)
+            tty.msg(f'Writing template {template_name} to {expand_path}')
 
             with open(expand_path, 'w+') as f:
                 f.write(self.expander.expand_var(template_conf['contents']))