Adds a local or global configuration file to OpenQML

[josh146]

This pull request adds a simple configuration file to OpenQML. Changes include:

1. the new Configuration class in configuration.py, to load, store, and save configuration information.
2. on OpenQML initialisation __init__.py, certain directories are now scanned, and if a config.toml file is found, it is loaded up to be the default configuration. The three directories are:
   a. The current/script execution directory
   b. The directory under environment variable OPENQML_CONF
   c. The standard user configuration directory (i.e. ~/.config/openqml on linux, %APPDATA%\openqml on windows, etc.)
3. the openqml.device loader now also accepts a Configuration object as a keyword argument. If not provided, the default_config determined in (2) is used. These are merged with the provided kwargs and used to load the device.

Two new modules are required: toml and appdirs. TOML was chosen as it is simpler than JSON or YAML (it is based on ini), and standardised (supports comments, nesting, etc.). The appdirs module allows easy determination of standard user configuration directories across all operating systems.

I've also added a default_config.toml file to the repository for reference. Since TOML allows nesting, you can set config options either (a) globally, (b) for all devices within a plugin, or (c) for only a specific device.

---

Note: it should be up to the OpenQML documentation, as well as all plugin documentation, to clearly set out the configurations allowed in the config file.

[co9olguy]

Looks good. Any way we could add a test (which runs on a default config file) which makes sure the Configuration class and loading from config works as expected?