This reverts commit e8503e28fd.
This imposed too many limitations on the renderer implementation. E.g.
making it impossible to store `Event<'s>`'s in the renderer struct.
Revert back to having the renderer struct separate from the implementor
of the Render trait. The implementor may instead create a renderer
struct without any restrictions.
This commit also adds a help text, accessible with the `--help` flag, as
well as a version text, available by using `--version`. My hope is that
this commit will make the jotdown CLI a bit friendlier to use.
