Overwrite Option

[marwanhilmi]

Hey @maxogden,

I created this as potential fix to #20. By default overwrite is true but we can toggle it off. I created a function to remove the symlink if it already exists. I added some tests but I did run into a weird issue with the last test:

test('callback called once', function (t) {
  rimraf.sync(targetC)
  t.plan(2)
  console.log('extracting to', targetC)
  extract(sourceC, {dir: targetC}, function (err) {
    if (err) throw err

    // this triggers an error due to symlink creation
    // NOTE the new overwrite: false option
    extract(sourceC, {dir: targetC, overwrite: false}, function (err) {
      if (err) t.ok(true, 'error passed')
      t.ok(true, 'callback called')
    })
  })
})

It craps out with:

events.js:141
      throw er; // Unhandled 'error' event
      ^
Error: EBADF: bad file descriptor, read
    at Error (native)

Any ideas?
Looks like this unresolved node bug: nodejs/node#2955 && nodejs/node#2950

[marwanhilmi]

Instead of doing a path exists check before attempting to delete, I just try to delete and if there's a ENOENT, we just proceed to write the symlink.

[max-mapper]

Crazy, haven't run into this case before. cc'ing @mafintosh here too

[unindented]

@maxogden any intent to merge the PR?