Revert "Create .add.extension(extenstion, reader) function for supporting external readers"

This reverts commit 5a308205299b2498eca4389e256b793b145a333c.

Conflicts:
	DESCRIPTION
	NEWS.md
	R/extensions.dispatch.table.R
	tests/7.R
Merge branch 'v0.5-rc' into v0.5-rc-full