This patch adds a new --viewer command line option that lets the user
select a viewer that displays the documentation. It also adds a default
viewer implementation that uses html2text to generate plain text from
the HTML documentation.
html2text requires the ptr_cast feature and we use the Option::as_deref
method in the viewer implementation, so we have to update the minimum
supported Rust version to 1.40.
This patch adds the parser module that uses kuchiki and html5ever to
parse the HTML documentation.
As kuchiki requires std::mem::MaybeUninit, we have to bump the minimum
supported Rust version to 1.36.
