Implemented support for per-buffer configuration

- Enabled taggatron functionality on a per buffer basis: all configuration options can now be set either as global or as buffer variable (buffer variables will override globals).

- Optimised initialisation speed: Placed all functions required to initialise taggatron into plugin/taggatron.vim. All other functions are placed into autoload/taggatron.vim and are only loaded when the plugin is executed for the first time.

- Optimised some of the functions to exit at the earliest possible point

- Improved code comments