Add commands to buy phone numbers

[childish-sambino]

Fixes

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Contributing to Twilio

All third party contributors acknowledge that any contributions they provide will be made under the same open source license that the open source project is provided under.

Short description of what this PR does:

• Adds the phone-numbers:buy topic and commands:

> twilio phone-numbers:buy
purchase Twilio phone numbers

USAGE
  $ twilio phone-numbers:buy:COMMAND

COMMANDS
  phone-numbers:buy:local               purchase a local phone number
  phone-numbers:buy:machine-to-machine  purchase a machine-to-machine phone number
  phone-numbers:buy:mobile              purchase a mobile phone number
  phone-numbers:buy:national            purchase a national phone number
  phone-numbers:buy:shared-cost         purchase a shared-cost phone number
  phone-numbers:buy:toll-free           purchase a toll-free phone number
  phone-numbers:buy:voip                purchase a voip phone number

If you have questions, please file a support ticket, or create a Github Issue in this repository.

[codecov]

Codecov Report

Merging #38 into master will increase coverage by 0.66%.
The diff coverage is 97.9%.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
+ Coverage    98.4%   99.06%   +0.66%     
==========================================
  Files          22       24       +2     
  Lines         565      644      +79     
==========================================
+ Hits          556      638      +82     
+ Misses          9        6       -3

Impacted Files	Coverage Δ
src/commands/phone-numbers/list.js	100% <ø> (ø)	⬆️
src/base-commands/twilio-api-command.js	97.56% <100%> (+5.42%)	⬆️
src/commands/projects/add.js	97.82% <100%> (ø)	⬆️
src/commands/phone-numbers/update.js	100% <100%> (ø)	⬆️
...esource-helpers/api/v2010/incoming-phone-number.js	100% <100%> (ø)	⬆️
src/services/twilio-api/index.js	100% <100%> (ø)	⬆️
src/hooks/init/buy-phone-number.js	97.26% <97.26%> (ø)
src/services/twilio-api/api-command-runner.js	98.3% <98.3%> (ø)

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[thinkingserious]

Nice simplifications throughout! 🥇