[FEATURE] Add multi-file selection to FileInput widget

[bruns01]

The ISSUE:
The current version (bokeh 1.4, branch master) of the FileInput widget of bokeh only allows a single file to be selected by the user.
The underlying item in the browser comes with the option/property "multiple" in JS or HTML5, which is hard-coded to "False" in the bokeh-code.

The Problem:
For data analysis project in my laboratory I need to upload many tenth of files at a time, which is not manageable by single upload.
Hence the request (and a related PR) to add a working "multiple"-property and the respective option of multi-file selection to the FileInput widget.

Solution:
Adapt/extend the code of the currently implemented FileInput to support a "multiple"-property with the (switchable) capability to support the multi-file selection and provide the returned results as arrays instead of scalar values.

Background:
The issue was pre-discussed here in community support and here in the develeopment-forum.

possible Alternative:
The implementation of a seperate widget (MultiFileInput) was considered but rejected because of the inconsistency with usual HTML or JS input widgets of similar functionality.