Add support for ignoring files

Add ignore.toml, configuration for ignoring files
Add file_filter.py, for matching files against glob/regex patterns
Update relevant code to use file filter
+Tests

pr_agent/settings/ignore.toml

@@ -0,0 +1,5 @@
+[ignore]
+
+# Ignore files and directories matching these patterns.
+glob = []
+regex = []