Support custom search key list when searching

[springuper]

In this PR, I added another option keys in search method to be able to specify a list of key when performing index search:

fuse.search('od man', { keys: ['title'] }) // returns 1 item

fuse.search('od man', { keys: ['tags'] }) // returns an empty list

It's quite useful in such scenarios:

• When there is a lot of noise in the all-keys results, and there is a key filter in the search bar so users could specify which keys to search
• When we only care about one or few keys, especially we hope the ordering should be based on those specified keys rather than all

[springuper]

@krisk hopefully you could help take a look at this PR in the near future.