entropy account export

[mixmix]

We have entropy account import but not a tool which allows export!

This needs design, but some considerations are:

• do we offer nmeumonics ... if so then we need to be able to import those too
• is the resulting output written to a file or stdout?
• do we need to support derivation paths (see Keyring)

See also:

• As a user, when I generate a new key, I'd like to have all information about my newly created key #82
• exporting and importing wallet files #21

[mixmix]

we could also offer entropy account ls -l (--long) which outputs the seed / mnemonic as well