Adding ignore option to ignore a list of elements during the sync

[leesei]

Add exclude option
Use exec() instead of spawn() to avoid parameter escaping

@tommy351 @oncletom closing original PR
hexojs/hexo#246

[raphaklaus]

Is this on the roadmap? I don't know if it's related but when I run the deploy routine using a ssh key with a passphrase I've got permission denied message.

Running manually the command generated by deployer.js in the spawn function, I'm asked for the passphrase and the deploy works normally.