UX: Parse recipients as filenames last instead of first

[str4d]

Currently, rage tries parsing a -r/--recipient argument as a filename first. If that file exists, then age will read it as a list of recipients (one per line, ignoring comments and blank lines), returning an error on any non-recipient content. If the file does not exist, then it tries other formats (which are all string-prefixed). However, basically all the string-prefixed formats (excluding e.g. HTTPS URLs) are themselves valid filenames. While it is unlikely that a user would have filenames named after age keys, it is possible that e.g. the GitHub notation (currently github:username, might be changing to @username) could be a filename. The only way around this currently is to move or rename the file, which is not intuitive.

Instead, let's parse the recipient as a filename last. Then if a user really wants to open a filename that is also itself a recipient key, they can simply use an absolute path.