[Fix #12] Support custom external reporters

[stanleyhlng]

This PR supports custom external reporters. Example: mocha-junit-reporter

config

➜  mocha-multi-reporters-demo git:(mocha-junit-reporter) ✗ cat config.json
{
    "reporterEnabled": "tap,xunit,mocha-junit-reporter",
    "xunitReporterOptions": {
        "output": "xunit-custom.xml"
    },
    "mochaJunitReporterReporterOptions": {
    "mochaFile": "junit-custom.xml"
    }
}

tap reporter result

➜  mocha-multi-reporters-demo git:(mocha-junit-reporter) ✗ npm test

> mocha-multi-reporters-demo@1.0.9 test /Users/sng/Projects/stanleyhlng/github/mocha-multi-reporters-demo
> mocha --reporter mocha-multi-reporters --reporter-options configFile=config.json

1..4
ok 1 mocha-test 1 sample test 1.1
ok 2 mocha-test 1 sample test 1.2
ok 3 mocha-test 2 sample test 2.1
ok 4 mocha-test 2 sample test 2.2 # SKIP -
# tests 3
# pass 3
# fail 0

mocha-junit-reporter reporter result

➜  mocha-multi-reporters-demo git:(mocha-junit-reporter) ✗ cat junit-custom.xml
<?xml version="1.0" encoding="UTF-8"?>
<testsuites name="Mocha Tests" time="0.002" tests="4" failures="0" skipped="1">
  <testsuite name="Root Suite" timestamp="2016-10-30T00:54:52" tests="0" failures="0" time="0">
  </testsuite>
  <testsuite name="mocha-test #1" timestamp="2016-10-30T00:54:52" tests="2" failures="0" time="0.002">
    <testcase name="mocha-test #1 sample test #1.1" time="0.001" classname="sample test #1.1">
    </testcase>
    <testcase name="mocha-test #1 sample test #1.2" time="0.001" classname="sample test #1.2">
    </testcase>
  </testsuite>
  <testsuite name="mocha-test #2" timestamp="2016-10-30T00:54:52" tests="2" failures="0" time="0">
    <testcase name="mocha-test #2 sample test #2.1" time="0" classname="sample test #2.1">
    </testcase>
  </testsuite>
</testsuites>%

xunit reporter result

➜  mocha-multi-reporters-demo git:(mocha-junit-reporter) ✗ cat xunit-custom.xml
<testsuite name="Mocha Tests" tests="4" failures="0" errors="0" skipped="1" timestamp="Sun, 30 Oct 2016 00:54:52 GMT" time="0.007">
<testcase classname="mocha-test #1" name="sample test #1.1" time="0.001"/>
<testcase classname="mocha-test #1" name="sample test #1.2" time="0.001"/>
<testcase classname="mocha-test #2" name="sample test #2.1" time="0"/>
<testcase classname="mocha-test #2" name="sample test #2.2" time="0"><skipped/></testcase>
</testsuite>

[coveralls]

Coverage decreased (-5.2%) to 87.143% when pulling 8ee1d05 on external-reporters into 867306e on master.