added support for global prefix

[jjanczyszyn]

Adds support for FASTLOG_GLOBAL_PREFIX environment variable

e.g.

process.env.FASTLOG_GLOBAL_PREFIX = '[ipAddress]';
var logger = require('fastlog')('configured', 'error', '${level} [${ category }] <${timestamp}>');
logger.error('You\'ve got a friend in me.');
// [ipAddress] error [configured] <Fri, 05 Dec 2014 02:10:48 GMT> You've got a friend in me.

why this might be useful:

Currently there's no way to ensure that all the logs in your application are using the same prefix.
For instance when you're including a dependency that's logging warnings (assuming that it's using fastlog for that). With the use of FASTLOG_GLOBAL_PREFIX it's easy.

For my use case, I'd like to include the EC2 ip address and be able to query logs for different instances later on.

[jjanczyszyn]

not sure why Travis is failing.
I checked locally and when running node v0.10.48 (the version that travis is using) I get the same error on master.

the error:

/home/travis/build/willwhite/fastlog/node_modules/mocha/node_modules/debug/src/node.js:120
exports.inspectOpts = Object.keys(process.env).filter(key => {
                                                           ^
SyntaxError: Unexpected token >
    at Module._compile (module.js:439:25)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Module.require (module.js:364:17)
    at require (module.js:380:17)
    at Object.<anonymous> (/home/travis/build/willwhite/fastlog/node_modules/mocha/node_modules/debug/src/index.js:9:19)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
npm ERR! Test failed.  See above for more details.
The command "npm test" exited with 1.