Have `filepicker` accept default observables

[ranjanan]

Firstly, thank you for such a useful package! This package greatly simplifies writing Web-based UIs . 😄

I was hoping to make a simple contribution. This PR modifies filepicker to take in a default observable, much like checkbox, for example.

[piever]

Thanks! This was something I had forgotten to do. I left a few comments, in particular:

• make sure tests pass
• make sure it works for any sensible initial value the user may give (nothing, a string or a Observable).

[piever]

I actually like the idea of having the observable start at nothing and in the @manipulate the @manipulate for x = nothing could create a file selection widget. Say:

@manipulate for x = nothing
  x != nothing && histogram(column(loadtable(x), 1))
end

[codecov-io]

Codecov Report

Merging #36 into master will decrease coverage by 0.85%. The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master      #36      +/-   ##
==========================================
- Coverage   68.11%   67.25%   -0.86%     
==========================================
  Files          13       13              
  Lines         276      281       +5     
==========================================
+ Hits          188      189       +1     
- Misses         88       92       +4

Impacted Files	Coverage Δ
src/input.jl	86.32% <33.33%> (-2.97%)	:arrow_down:

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[ranjanan]

The tests pass and it works for all sensible inputs:

julia> using InteractBase

julia> filepicker(value = "hello")
(div { className="interact-widget" }
    (input { attributes=Dict{Any,Any}(Pair{Any,Any}("class", "interact-widget "),Pair{Any,Any}("type", "file"),Pair{Any,Any}("v-on:change", "onFileChange"),Pair{Any,Any}("ref", "data")) }))

julia> filepicker()
(div { className="interact-widget" }
    (input { attributes=Dict{Any,Any}(Pair{Any,Any}("class", "interact-widget "),Pair{Any,Any}("type", "file"),Pair{Any,Any}("v-on:change", "onFileChange"),Pair{Any,Any}("ref", "data")) }))

julia> using Observables

julia> filepicker(value = Observable{Any}("hello"))
(div { className="interact-widget" }
    (input { attributes=Dict{Any,Any}(Pair{Any,Any}("class", "interact-widget "),Pair{Any,Any}("type", "file"),Pair{Any,Any}("v-on:change", "onFileChange"),Pair{Any,Any}("ref", "data")) }))

[piever]

Nice job!