PGP decrypt: automatically wrap with "BEGIN PGP MESSAGE" when no header is specified

Many tools that output PGP messages (e.g. Terraform's aws_iam_access_key) skip the header. Rather than printing this:
```
-----BEGIN PGP MESSAGE-----
Version: Keybase OpenPGP v2.0.73
Comment: https://keybase.io/crypto

wc903ksdf98032idso903= (not actual data)
-----END PGP MESSAGE-----
```
They'll print this instead:
```
wc903ksdf98032idso903= (not actual data)
```
The latter is quite hard to decrypt with Keybase. I tried numerous ways of getting this decrypted (e.g. piping with `base64 -d`, etc) and it seems the only way to get it done is to wrap the string with the header. It applies both to `keybase pgp decrypt` and https://keybase.io/decrypt.

It'd be great if Keybase could automatically wrap the encrypted message with the header to avoid the hassle of doing it manually. (Or having to pass multi-line strings that are error-prone when pasting around)

The issue names the keybase pgp decrypt command and the keybase.io/decrypt page; start by locating their decryption entry points and existing armored-message handling. Done means both paths accept the described headerless encrypted text and decrypt it without manual wrapping, with coverage for that input.

Written by the indexing model from the issue text.