Change config helper to use sync and have config per command

[tomdye]

• sync not async because register command where this could be used is sync.
• change API to take a commandName to avoid config conflicts between commands.
• checking ejected state in eject command before ejecting.
• change config helper to expose get / save functions and not be a class.
• check ejected state before loading a command.

Blocks: https://github.com/dojo/cli-build/issues/58 Fixes: https://github.com/dojo/cli/issues/112 #104 #106

[codecov-io]

Codecov Report

Merging #110 into master will increase coverage by 0.01%. The diff coverage is 97.5%.

@@            Coverage Diff             @@
##           master     #110      +/-   ##
==========================================
+ Coverage   99.43%   99.45%   +0.01%     
==========================================
  Files          15       15              
  Lines         357      364       +7     
  Branches       38       40       +2     
==========================================
+ Hits          355      362       +7     
  Partials        2        2

Impacted Files	Coverage Δ
src/commands/eject.ts	100% <100%> (ø)	:white_check_mark:
src/CommandHelper.ts	100% <100%> (ø)	:white_check_mark:
src/configurationHelper.ts	100% <100%> (ø)
src/registerCommands.ts	100% <100%> (ø)	:white_check_mark:
src/loadCommands.ts	97.29% <92.3%> (+0.42%)	:white_check_mark:

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[tomdye]

@agubler this is ready for review now. Fully working and tested.