chore: add test and docs for high and low watermarks (#493)

README.md

@@ -312,6 +312,31 @@ nyc
     └── /usr/local/bin/node /path/to/your/project/node_modules/ava/lib/test-worker.js …
 ```
 
+## High and low watermarks
+
+Several of the coverage reporters supported by nyc display special information
+for high and low watermarks:
+
+* high-watermarks represent healthy test coverage (in many reports
+  this is represented with green highlighting).
+* low-watermarks represent sub-optimal coverage levels (in many reports
+  this is represented with red highlighting).
+
+You can specify custom high and low watermarks in nyc's configuration:
+
+```json
+{
+  "nyc": {
+    "watermarks": {
+      "lines": [80, 95],
+      "functions": [80, 95],
+      "branches": [80, 95],
+      "statements": [80, 95]
+    }
+  }
+}
+```
+
 ## Integrating with coveralls
 
 [coveralls.io](https://coveralls.io) is a great tool for adding

test/src/nyc-bin.js

@@ -689,4 +689,39 @@ describe('the nyc cli', function () {
       done()
     })
   })
+
+  it('allows alternative high and low watermarks to be configured', function (done) {
+    var args = [
+      bin,
+      '--watermarks.lines=90',
+      '--watermarks.lines=100',
+      '--watermarks.statements=30',
+      '--watermarks.statements=40',
+      '--cache=true',
+      process.execPath,
+      './half-covered.js'
+    ]
+
+    var proc = spawn(process.execPath, args, {
+      cwd: fixturesCLI,
+      env: {
+        PATH: process.env.PATH,
+        FORCE_COLOR: true
+      }
+    })
+
+    var stdout = ''
+    proc.stdout.on('data', function (chunk) {
+      stdout += chunk
+    })
+
+    proc.on('close', function (code) {
+      code.should.equal(0)
+      // 50% line coverage is below our low watermark (so it's red).
+      stdout.should.match(/\[91m\W+50\W+/)
+      // 50% statement coverage is above our high-watermark (so it's green).
+      stdout.should.match(/\[92m\W+50\W+/)
+      done()
+    })
+  })
 })