Add shorthand accountDID method

[icidasset]

Closes #443

program.accountDID(username)

I thought about doing just program.accountDID() without the username param which would depend on the session underneath and return null if not authed. But seeing that loadFileSystem also takes the username parameter, it makes sense to implement it that way.

[bgins]

Looks good! 🎉

[avivash]

ahh nice, looks good!