Feat: refactor to enable additional bridge

[stanleyyconsensys]

Ledger is a hardware wallet that support multiple type of integration, it can be integrate by web or by mobile (thought bluetooth)

The current eth-ledger-bridge-keyring is only support web base by using iframe, and the class LedgerKeyring is hardcoded the bridge URL to be passed to lower level bridge

Hence, this PR is focus on enable multi bridge to be integrated into ledger keyring, and moving the bridge specific configuration/parameter into the bridge itself, thus to make the ledger keyring unify

• adding setOptions, getOptions method to configure bridge setting on the fly
• move bridgeUrl from LedgerKeyring to LedgerIframeBridge as configuration options, hence LedgerKeyring class remain universal to support different type of bridge

[legobeat]

How does this relate to #198?

[stanleyyconsensys]

198

Hi @legobeat thanks for reviewing, the PR is the decouple PR for moving the bridgeURL into iframe bridge itself hence, i will create another PR that focus on mobile bridge, how does it sounds to you?

i will close #198 anyway

[stanleyyconsensys]

@cryptodev-2s @mikesposito can you merge for me, i dont have the permission to do so