Documentation for all vars

Tagging v1.3.1 for release

The following are the major changes that went through in this release

Fix: BUNDLED_AVR_TOOLS_DIR is now set properly when using only arduino-core and not the whole arduino package. (https://github.com/sej7278)
New: Document all variables that can be overridden. (https://github.com/sej7278)
New: Add a new help_vars target to display information about variables that can be overridden.

Major credit goes to @sej7278