Support multiple config files

Resolves #189

* configuration formatting and comments are preserved after editing with `tronfig`
* trond now accepts a config_path option instead of a config_file
* adding `tools/migration/migrate_config_0.5.1_to_0.5.2.py` to migrate old config files to the new multi-file format
* `tronfig` now accepts a `config_name` argument which can be used to create and update named configurations (defaults to `MASTER`)
* named configurations are now checked for valid node/node_pool names
* when editing a named configuration, a header is added to the config contents so that users know which nodes and command context are available to them. The header is removed before saving the config contents.