Add `asset.ignore` option

[samuelmeuli]

Hi,

Would you be open to an ignore options for assets? This would e.g. simplify copying entire directories except for a few files, as it wouldn't be required to write a complicated glob pattern for that.

The value of the ignore option can simply be passed to functions of the glob module (see https://github.com/isaacs/node-glob#options).

[ifahrentholz]

Thanks for your pull request. The ignore option is working fine I've checked it manually.. Since we want automatic tests for these kind of changes an update of our TestCase script seems to be necessary.

[samuelmeuli]

I've added a test.

[ifahrentholz]

Thanks for your contribution :)