fix: Log errors in the background subprocess

Currently there is not output if the background subprocess exits with an error,
for example because the configuration file has a syntax error. This commit fixes
that.
Merge pull request #116 from mzgol/error-handling

fix: Log errors in the background subprocess
docs(readme): svg badge
Merge pull request #121 from sahat/patch-1

docs(readme): svg badge
fix: Pass node-specific command line params to the subprocess

This commit makes the karma task pass Node-specific parameters used to start
the Node process to its background subprocess. This is required e.g. to be able
to use the --harmony flag.

Similar pull request for Grunt itself (already merged):
https://github.com/gruntjs/grunt/pull/877
Merge pull request #117 from mzgol/node-args

fix: Pass node-specific command line params to the subprocess
feat: Use this.files, let Grunt do the file matching.

Use the task's `files` property, but merge with `options.files`,
so we can specify some files that should be available in all test-targets.
Grunt's file objects may be extended with the file options supported
by Karma (included, watched).
Projects that use the undocumented `karma.<target>.files` option should
rename this option and use `karma.<target>.src` instead.