Make IO Context available inside `<script>`

[fonsp]

This PR makes it possible to use get(io, :key, "default") inside custom show methods to query the io context of the renderer. This already worked for objects interpolated inside HTML, because this rendering is not proxied, but not for objects interpolated inside <script>.

# Set up a custom javascript-renderable type
struct Hello end

function Base.show(io::IO, ::MIME"text/javascript", ::Hello)
    # the rendered content depends on the IO context
    print(io, get(io, :hello, "oops"))
end

# Example usage:
h = @htl("""
<script>
const x = $(Hello())
</script>
""")

# Let's look at the output:
repr(
    MIME"text/html"(), h; 
    context=(:hello => "world")
)
#-> "<script>const x = world</script>"

[codecov-commenter]

Codecov Report

Merging #27 (6e85aec) into master (7bbabcc) will decrease coverage by 0.67%. The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master      #27      +/-   ##
==========================================
- Coverage   95.50%   94.82%   -0.68%     
==========================================
  Files           7        7              
  Lines         690      696       +6     
==========================================
+ Hits          659      660       +1     
- Misses         31       36       +5     

Impacted Files	Coverage Δ
src/script.jl	91.45% <ø> (ø)
src/style.jl	69.56% <ø> (ø)
src/primitives.jl	88.33% <16.66%> (-7.97%)	:arrow_down:

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