Added helper compress function.

[sharoninator]

This is a helper function for more shorthand compression. It detects whether the data its compressing is a file or a directory, and uses the appropriate functions based on that. Here's a usage example:

const archive = archiver('zip');
archive.compress('dir/', 'archive.zip');

fs is already a dependency, so it skips the step of manually passing in the file input stream.

Without this helper function, say I want to compress a directory. In the example, I am to do it as so:

// require modules
const fs = require('fs');
const archiver = require('archiver');

// create a file to stream archive data to.
const output = fs.createWriteStream(__dirname + '/example.zip');
const archive = archiver('zip', {
  zlib: { level: 9 } // Sets the compression level.
});

// pipe archive data to the file
archive.pipe(output);

// append files from a sub-directory and naming it `new-subdir` within the archive
archive.directory('subdir/', 'new-subdir');
archive.finalize();

This is quite verbose, just to compress a directory. This function makes it so it can be done in one line.