Fetch nonce automatically in transaction builder

[yknl]

This PR updates the transaction builder functions to automatically fetch the account nonce from a Stacks node if a nonce is not specified. This saves a step and makes integration easier. Nonce can be set manually in the options arg.

Resolves #73

Type of Change

• [x] New feature
• [ ] Bug fix
• [ ] API reference/documentation update
• [ ] Other

Does this introduce a breaking change?

No

Are documentation updates required?

Readme updated.

Testing information

Nonce should be fetched automatically in transaction builder when not specified via the options object.

Checklist

• [x] Code is commented where needed
• [x] Unit test coverage for new or modified code paths
• [x] npm run test passes
• [ ] Changelog is updated
• [ ] Tag 1 of @yknl, @zone117x, @reedrosenbluth for review

[codecov[bot]]

Codecov Report

Merging #74 into feature/option-arg-builders will decrease coverage by 0.56%. The diff coverage is 63.88%.

@@                       Coverage Diff                       @@
##           feature/option-arg-builders      #74      +/-   ##
===============================================================
- Coverage                        85.47%   84.90%   -0.57%     
===============================================================
  Files                               26       26              
  Lines                             1404     1438      +34     
  Branches                           252      262      +10     
===============================================================
+ Hits                              1200     1221      +21     
- Misses                             203      215      +12     
- Partials                             1        2       +1     

Impacted Files	Coverage Δ
src/utils.ts	73.43% <ø> (ø)
src/builders.ts	71.50% <50.00%> (-3.82%)	:arrow_down:
src/authorization.ts	76.96% <100.00%> (+0.26%)	:arrow_up:
src/keys.ts	95.23% <100.00%> (+0.23%)	:arrow_up:
src/network.ts	100.00% <100.00%> (ø)
src/transaction.ts	79.06% <100.00%> (+0.49%)	:arrow_up:

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