Support rollup-plugin-multi-input. Add optional no hash option.

[txantslusam]

Hi! I am in process of building library components. I decided to use rollup and a set of plugins including rollup-plugin-rebase to manage assets and rollup-plugin-multi-input to parse code. It appears that these two plugins conflicting between each other and in result, assets paths points to non existing directories.

eg.

import asset from '../../components/example/assets/asset.jpg'

instead of

import asset from '../../assets/asset.jpg'

For every entry, root was changing and in result it ends with folder of last file to parse.

With this change, plugin can detect, if we are using rollup-plugin-multi-input. Then root is setting only once at start.

Second thing is that I added option to optionally disable hashing of filenames, so they can remain the same (useful for libraries)

[txantslusam]

Closed in favour of #46 and #47