Add support for YAML metadata parsing

[bumi]

This allows loading a specified key from a as yaml formatted metadata part of a secret. (related issue: #112 )

Usage:

assuming the following secret:

password

yaml: mammal
name: Johne Doe

Display the value of a specified key:

gopass show yaml-example --key [yaml key]

Copy the value of a specified key:

gopass show yaml-example -k [yaml key] -c 

When the -k/--key option is given the first line (the password) of a secret will be ignored and the rest will be loaded using simpleyaml. The --clip and --qr flags are still supported and the value the --key will be used.

Editing the metadata part is currently not supported. In my usecase editing would mostly be manual and yaml is easy enough to write. So I don't think that this is super important.

I am opening this PR for discussion on how this feature should be implemented.
This is my first piece of go code that I am writing... so I don't really know what I am doing yet :)

ToDo:

• support any type of value in the YAML (currently strings are assumed)

[dominikschulz]

Thanks a lot for the PR.

At first sight this looks like going into the right direction, but still lacking a lot in terms of UX. Will give this some more thought / comments soon.

[bumi]

thanks, looking forward to your thoughts.

[dominikschulz]

Closing in favour of #125

Thank you very much for this PR. If this was your first attempt in Go it's definitely very good. We wanted to some things a little different and also include write support so I thought it would be easier if we just pick your approach and implement it directly.

[bumi]

whooot! awesome! will check it out. thanks!