Load configuration (from JSON)

[henrykrumb]

Make the following parameters configurable via JSON:

• Sensitivity per axis (x, y, z, ax, ay, az)
• Lock axes (true / false)
• Default IGTLink host and port

There should be a hierarchy of config files being loaded:

1. Load project-local config
2. Load system-wide config in $PREFIX/share/spacenav-igt/config.json
3. Load user config in ~/.config/spacenav-igt/config.json