Move genflags tool to cmd/ and pin to previous release

to alleviate problems caused by the circular dependency of using the
same code as a library that is potentially being generated to adhere to
a different API.

.gitignore

@@ -6,5 +6,6 @@ internal/*/built-example
 coverage.txt
 /.local/
 /site/
+/cmd/urfave-cli-genflags/urfave-cli-genflags
 
 *.exe