feat(cli): improvements and external commands

[galvez]

An attempt at implementing the universal approach from nuxt/rfcs#11

(edited by @pi0)

Main objectives

• Alias nuxt-custom ... commands with nuxt custom ... shortcut. Allowing a universal way for pluggable nuxt module commands.
• Expose NuxtCommand class (@nuxt/cli) for command authors

Other changes

• Internal refactors and test improvements
• Prevent process.exit as much as possible (Thanks to @pimlie tip)
• Refactors on NuxtCommand for making it ready for external commands
  • This class could be finally moved into module-utils package
• Refactors on built-in commands
• -v alias for version

[codecov-io]

Codecov Report

Merging #4314 into dev will decrease coverage by 0.49%.
The diff coverage is 62.5%.

@@            Coverage Diff            @@
##              dev    #4314     +/-   ##
=========================================
- Coverage   91.02%   90.53%   -0.5%     
=========================================
  Files          61       61             
  Lines        2196     2198      +2     
  Branches      527      529      +2     
=========================================
- Hits         1999     1990      -9     
- Misses        179      189     +10     
- Partials       18       19      +1

Impacted Files	Coverage Δ
packages/cli/src/options/common.js	66.66% <ø> (ø)	⬆️
packages/cli/src/commands/help.js	0% <0%> (ø)	⬆️
packages/cli/src/list.js	0% <0%> (ø)	⬆️
packages/cli/src/commands/start.js	66.66% <100%> (ø)	⬆️
packages/cli/src/commands/dev.js	79.16% <100%> (ø)	⬆️
packages/cli/src/commands/build.js	83.33% <100%> (+1.51%)	⬆️
packages/cli/src/commands/generate.js	83.33% <100%> (+4.76%)	⬆️
packages/cli/src/commands/index.js	11.11% <11.11%> (-88.89%)	⬇️
packages/cli/src/run.js	33.33% <37.5%> (-25.5%)	⬇️
packages/cli/src/utils/index.js	57.81% <80%> (ø)	⬆️
... and 2 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3fd059d...fb5821b. Read the comment docs.

[galvez]

Thanks @pi0 -- love the changes, LGreatTM :)