fix: missing command temp-directory (#928)

* Fix missing command temp-directory

After changing tempDirectory to tempDir, with a fallback for tempDirectory, the default values set in the 'temp-dir' yargs option were being used rather than falling back to tempDirectory and then the default temp dir.
This change removes the yargs default value for the option 'temp-dir'.
This means the statement that assigns '_tempDirectory' in 'index.js' attempts to use temp-dir, then temp-directory, then the default value.  
This is now under test with some new test cases that show:
  - tempDir is preferred to tempDirectory
  - tempDirectory is used if tempDir isn't set
  - that we fall back to the '.nyc_output' dir if neither tempDir or tempDirectory are set.

* Alternate implementation

Maintains notice of default temp-dir in help output.
Removed tests as I'm not confident with these right now, will take another look when I get some time.

Fixes #927.

index.js

@@ -41,7 +41,7 @@ function NYC (config) {
   this.config = config
 
   this.subprocessBin = config.subprocessBin || path.resolve(__dirname, './bin/nyc.js')
-  this._tempDirectory = config.tempDir || config.tempDirectory || './.nyc_output'
+  this._tempDirectory = config.tempDirectory || config.tempDir || './.nyc_output'
   this._instrumenterLib = require(config.instrumenter || './lib/instrumenters/istanbul')
   this._reportDir = config.reportDir || 'coverage'
   this._sourceMap = typeof config.sourceMap === 'boolean' ? config.sourceMap : true