Add ElectrumX merkle branch method

[JeremyZeng77]

Issue

Fixes #7.

Depends on Permissionless-Software-Foundation/bch-api#236 for the REST endpoint added by Permissionless-Software-Foundation/bch-api#9.

Summary

• adds Electrumx.merkleBranch(txid, height) for single transaction GET requests
• supports bulk merkle branch requests via an array of txid/height objects
• adds api-doc examples for single and bulk usage
• adds unit mocks, unit tests, and integration coverage

Verification

• git diff --check
• node --check src/electrumx.js
• node --check test/unit/electrumx.js
• node --check test/integration/electrumx.js
• node .\node_modules\standard\bin\cmd.cjs --env mocha src\electrumx.js test\unit\electrumx.js test\unit\fixtures\electrumx-mock.js test\integration\electrumx.js
• RESTURL=http://localhost:5942/v6 node .\node_modules\mocha\bin\mocha.js --timeout 30000 --grep "#merkleBranch" test\unit\electrumx.js: 3 passing
• RESTURL=http://localhost:5942/v6 node .\node_modules\mocha\bin\mocha.js --timeout 30000 test\unit\electrumx.js: 30 passing

Payout

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