Update find_unique_account to do substring matches

What does everyone think of this? There is likely better ways to do this or make it a command line option or configurable, but I thought I'd get everyone's feedback.

I made a tiny update to find_unique_account to match on unique substrings. The existing code already checks to see if the match is unique or not, so this allows for a bit simpler searching, and fits my mental model a bit more. Thoughts?

Signed-off-by: Michael Swieton <mike@swieton.net>
As this is not an exact matching anymore, I think it would be helpful to [list the matched accounts here](https://github.com/lastpass/lastpass-cli/pull/38/files#diff-553301f575178a52d23adfd226bfadb2R106)