fix: respect options.cwd

First, thank you for this module. :+1:

Please consider the following directory structure:
```
src/
    a.js
    ...
dest/
```
One may try to copy all `*.js` below `src` to `dest`:
```
src/
    a.js
    ...
dest/
    a.js
    ...
```
but currently using
```
cpy(['*.js'], 'dest', {cwd: 'src'}, cb);
```
throws an ENOENT-Error because `a.js` cannot be found within the `process.cwd` directory.
This PR fixes that issue.