Hotswapping

This release implements some extra source file handler functions (issues #4 and #5).

chatterbox_load_from_buffer() allows you to skip reading data from an external file and instead allows you to create source files from simple buffers. chatterbox_unload() will let you remove a source file from memory, and chatterbox_is_loaded() will return if the given source file is in memory.

There is also a change in behaviour for chatterbox_load() - if you try to load a source file using the same name as an existing source file, the old source file will be invalidated. This allows for hotswapping of chatterbox source files which makes iterating on dialogue much easier. Please note that you will need to manually restart any in-progress chatterboxes that are using the hotswapped source file. Hotswapping is also available when using the new chatterbox_load_from_buffer() function.

Additionally, this release also includes changes from the unreleased 0.5.1 version which are mostly bug fixes. However, "internal" variables are now called "yarn" variables to make their behaviour and purpose clearer. Please update your Yarn script accordingly.