feat: Add support for env.NYC_CONFIG_OVERRIDE (istanbuljs#1077)

This allows a process to spawn a child process with an environment
variable NYC_CONFIG_OVERRIDE.  This is a JSON string which overrides any
values in the NYC_CONFIG env.  It is not deleted in the child process,
so will be contagious unless unset, and it is the responsibility of the
caller to manage.

The first intended use case of this feature is to allow node-tap to map
tests to the specific portion of a system under test that they cover.
In this way, unit tests can be more focused, and only re-run when their
specific unit has changed.

There are, of course, many other uses that this could be put to, and it
did not seem appropriate to add a special hook _just_ for overriding
the include list.

bin/wrap.js

@@ -16,6 +16,12 @@ if (process.env.NYC_PROCESSINFO_EXTERNAL_ID) {
   delete process.env.NYC_PROCESSINFO_EXTERNAL_ID
 }
 
+if (process.env.NYC_CONFIG_OVERRIDE) {
+  var override = JSON.parse(process.env.NYC_CONFIG_OVERRIDE)
+  config = Object.assign(config, override)
+  process.env.NYC_CONFIG = JSON.stringify(config)
+}
+
 ;(new NYC(config)).wrap()
 
 sw.runMain()