index.js: Fallback to require(...) for unrecognized reporters

[wking]

This allows us to use reporters that aren't built in. For example, with this commit you can:

$ npm install mochawesome
$ tap --reporter=mochawesome ...

Fixes #46.

[wking]

I've pushed a number of other commits to this branch to get Mochawesome working. With the current tip (e4f8183), you can test with two dummy TAP generators:

$ cat /tmp/test1.sh 
#!/bin/sh
cat <<EOF
TAP version 13
ok 1 # SKIP testing a skip
not ok 2 - can multiply one by two
  ---
  {
    "actual": 3,
    "expected": 2
  }
  ...
ok 3 - can multiply two by two
  ---
  {
    "actual": 4,
    "expected": 4
  }
  ...
1..3
EOF
$ cat /tmp/test2.sh 
#!/bin/sh

cat <<EOF
TAP Version 13
1..2
# nesting
    1..2
    ok 1 - true is ok
    not ok 2 - doag is also okay
not ok 1 - nesting
ok 2 - second
EOF

Install tap and clobber the npm-installed tap-mocha-reporter with the content from this branch:

$ npm install
$ npm install --no-save mocha mochawesome
$ cp index.js node_modules/tap-mocha-reporter/
$ cp lib/*.js node_modules/tap-mocha-reporter/lib/

Test this repository, feeding the output into Mochawesome:

$ npm test -- --reporter=mochawesome

Test the dummy generators, feeding the output into Mochawesome:

$ node_modules/.bin/tap --reporter=mochawesome /tmp/test1.sh /tmp/test2.sh

For folks who are curious, but not curious enough to run the above themselves ;), you can see the generated output here while this PR is in flight.