Load data from files

[scionaltera]

There are several types of data that are loaded into the game with classes such as CommandLoader. These classes were a simple way to set up a default state but they're not very flexible. If you wanted to customize your MUD in a persistent way (eg. removing the diagonal movement commands) you'd have to make a code change and from that point on you would always be fighting against git trying to overwrite your changes when you merge from upstream.

A better way to do this would be to make the loader classes load from a file, where the file is not managed by git.

There are currently four loader classes:

• CommandLoader
• WorldLoader
• ProfessionLoader
• SpeciesLoader

For this issue we'll ignore WorldLoader because that's covered by #406 already.

For each of the three remaining three loaders make the following changes:

1. Define an appropriate file format. It should be human readable/editable. JSON might work but I don't like that it doesn't have comments. A custom format would be OK, or something like YAML or XML maybe?
2. Create a default file in src/main/resources/ to load some minimal defaults from if no other source is available.
3. If data exists in the database already, don't load anything from files (same as current behavior).
4. If a custom file exists under ./data/{filename}, load that.
5. If no custom file exists, load the default file from ./agonyforge-mud-demo/src/main/resources.
6. Add the custom files to .gitignore.