feat - add label search argument to sign-invoice

[VishnuJin]

fixes #299

this PR adds optional ability to sign-invoice with a key that matches the given label

For example

--label-matching (partial match)

❯ bindle sign-invoice -o signed-invoice.toml invoice.toml --label-matching "Vishnu Jin"
Signed invoice.toml with role creator, label as 'Vishnu Jin <vishnujin@outlook.com>' and wrote to signed-invoice.toml

--label (exact match)

❯ bindle sign-invoice -o signed-invoice.toml invoice.toml --label "Vishnu Jin <vishnujin@outlook.com>"
Signed invoice.toml with role creator, label as 'Vishnu Jin <vishnujin@outlook.com>' and wrote to signed-invoice.toml

[VishnuJin]

Also, it would be great if we could have a --label that does an exact match, but that is optional

yeah that was my original intention as well.I will implement it

[VishnuJin]

@thomastaylor312 can we use the -l as short for --label and -m as short for --label-match ?

[thomastaylor312]

Those sound like great shorthands

[VishnuJin]

@thomastaylor312 I have just modified the approach here, if you like this approach I will remove the earlier commits !! let me know your thoughts

[VishnuJin]

@thomastaylor312 this is ready for review now