fix: Report error if unwanted positional arguments are received

This applies to check-coverage, instrument, merge and report.  Passing
additional arguments will now cause the help script to be displayed and
an error exit code.

Unknown flags `nyc report --unknown=1` are still not reported.
Reporting unknown flags would require additional work as
`yargs.strict()` causes unknown items from configuration to be reported,
including flags that are defined for the global command but not a
sub-command.

Fixes #401

lib/commands/check-coverage.js

@@ -7,6 +7,7 @@ exports.describe = 'check whether coverage is within thresholds provided'
 
 exports.builder = function (yargs) {
   yargs
+    .demandCommand(0, 0)
     .option('exclude', {
       alias: 'x',
       default: testExclude.defaultExclude,

lib/commands/instrument.js

@@ -9,6 +9,15 @@ exports.describe = 'instruments a file or a directory tree and writes the instru
 
 exports.builder = function (yargs) {
   return yargs
+    .demandCommand(0, 0)
+    .positional('input', {
+      describe: 'file or directory to instrument',
+      type: 'text'
+    })
+    .positional('output', {
+      describe: 'directory to output instrumented files',
+      type: 'text'
+    })
     .option('require', {
       alias: 'i',
       default: [],

lib/commands/merge.js

@@ -11,6 +11,7 @@ exports.describe = 'merge istanbul format coverage output in a given folder'
 
 exports.builder = function (yargs) {
   return yargs
+    .demandCommand(0, 0)
     .positional('input-directory', {
       describe: 'directory containing multiple istanbul coverage files',
       type: 'text',

lib/commands/report.js

@@ -7,6 +7,7 @@ exports.describe = 'run coverage report for .nyc_output'
 
 exports.builder = function (yargs) {
   return yargs
+    .demandCommand(0, 0)
     .option('reporter', {
       alias: 'r',
       describe: 'coverage reporter(s) to use',

test/nyc-integration.js

@@ -49,7 +49,7 @@ t.test('--check-coverage fails when check-coverage command is used rather than f
   return testSuccess(t, {
     args: [process.execPath, './half-covered.js']
   }).then(() => testFailure(t, {
-    args: ['check-coverage', '--lines', '51', process.execPath, './half-covered.js']
+    args: ['check-coverage', '--lines', '51']
   }))
 })