all: temporarily disable testscript tests

They require a replace directive that breaks "go install". Will revert
this after tagging a new latest release.

internal/bech32: remove data length limit

Fixes FiloSottile#453

doc: regenerate groff and html man pages

README: resize and center the logo

age: move the scrypt lone recipient check out of Decrypt

The important one is the decryption side one, because when a user types
a password they expect it to both decrypt and authenticate the file.
Moved that one out of Decrypt and into ScryptIdentity, now that
Identities get all the stanzas. special_cases--

This also opens the door to other Identity implementations that do allow
multiple scrypt recipients, if someone really wants that. The CLI will
never allow it, but an explicit choice by an API consumer feels like
something we shouldn't interfere with.

Moreover, this also allows alternative Identity implementations that use
different recipient types to replicate the behavior if they have the
same authentication semantics.

The encryption side one is only a courtesy, to stop API users from
making files that won't decrypt. Unfortunately, that one needs to stay
as a special case in Encrypt, as the Recipient can't see around itself.
However, changed it to a type assertion, so custom recipients can
generate multiple scrypt recipient stanzas, if they really want.

armor: don't leave an empty line before the footer

Closes FiloSottile#264
Fixes FiloSottile#263

cmd/age: improve a couple error messages

age: make Identity and Recipient work on multiple stanzas

This is a breaking change, but like the other changes to these
interfaces it should not matter to consumers of the API that don't
implement custom Recipients or Identities, which is all of them so far,
as far as I can tell.

It became clear working on plugins that we might want Recipient to
return multiple recipient stanzas, for example if the plugin recipient
is an alias or a group. The Identity side is less important, but it
might help avoid round-trips and it makes sense to keep things
symmetric.

cmd/age: touch up help text

age: clean up examples of hardcoded data