add ability to cancel a single order using the order id

[definitelycarter]

• Add ability to cancel an order using the order id.
• Updated the jsdoc comment for cancelOpenOrders using the verbiage from the Coinbase API docs.

Fixes #260

[definitelycarter]

I'll add a test for this new method.

[codecov-io]

Codecov Report

Merging #283 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #283   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files           21        21           
  Lines          479       482    +3     
  Branches        29        29           
=========================================
+ Hits           479       482    +3     

Flag	Coverage Δ
#unittests	100.00% <100.00%> (ø)

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Impacted Files	Coverage Δ
src/order/OrderAPI.ts	100.00% <100.00%> (ø)

[bennycode]

Hi @definitelycarter and thank you for your contribution. I will merge it and add the optional parameter product_id to fully map out this endoint.

Thank you also for the description update of cancelOpenOrders. 👍