Support XDG

Hi,

I just discovered Hyperterm today and I'm really liking the idea of a web-based terminal (I use Atom as my code editor of choice). However, reading through the documentation, I noticed that configuring the editor (plugins and all) creates several files in your home directory (i.e. ~/.hyperterm.js and ~/.hyperterm_plugins). I also found issues on this repo like #10 and #14 which propose a possible custom stylesheet file (as well as the addition of an Extensions API).

I'd prefer not to clutter my home directory with more hidden files, if possible. Therefore, I feel that all of these files should be consolidated under a single ~/.hyperterm directory, similar to Atom's. It could have the following structure:

.hyperterm/
- plugins/
  - ...
- config.js
- styles.css (future)

...and whatever else might be added.

I figure to migrate current users to the new hierarchy, some backwards-compatibility code could be added which does the following:

1. Use .hyperterm for all configuration/plugins if the directory exists
2. If it does not exist, create a new .hyperterm directory and move ~/.hyperterm.js and ~/.hyperterm_plugins to their new respective paths

Thoughts? :) Caleb