Open()'ing binary files and multipart uploads

[robingustafsson]

This a continuation of #420 (also overriding #370).

It implements support for open()ing binary files and multipart uploads with the following API:

import http from "k6/http";

let binFile = open("/path/to/file.bin", "b");

export default function() {
    var data = {
        field: "this is a standard form field",
        file: http.file(binFile, "test.bin")
    };
    var res = http.post("https://example.com/upload", data);
}

[robingustafsson]

@spicykoala Here's the new PR :) I feel it's more or less ready to be merged but if you can give me feedback and/or test this PR that'd be great.

@na-- If you have any feedback on this PR I'd appreciate it as well.

[na--]

This escaping seems a bit strange. I think it's equivalent to strings.NewReplacer(`\`, `\\`, `"`, `\"`), but much harder to understand because of the mixed quotes. Also probably deserves a simple unit test just in case.

[na--]

Taking my words back since this is basically copied from https://golang.org/src/mime/multipart/writer.go?s=2899:3037#L126

[na--]

These strings should probably be escaped in the context they are used (line 124-125), not in the FileData struct "constructor"

[codecov-io]

Codecov Report

Merging #524 into master will decrease coverage by 0.2%.
The diff coverage is 39.39%.

@@            Coverage Diff             @@
##           master     #524      +/-   ##
==========================================
- Coverage   62.86%   62.66%   -0.21%     
==========================================
  Files          95       94       -1     
  Lines        6962     6835     -127     
==========================================
- Hits         4377     4283      -94     
+ Misses       2339     2302      -37     
- Partials      246      250       +4

Impacted Files	Coverage Δ
js/modules/k6/http/file.go	0% <0%> (ø)
js/initcontext.go	100% <100%> (ø)	⬆️
js/modules/k6/crypto/crypto.go	97.29% <100%> (ø)	⬆️
js/modules/k6/http/http_request.go	81.62% <24.32%> (-7.38%)	⬇️
lib/netext/dialer.go	42.42% <0%> (-31.11%)	⬇️
stats/thresholds.go	79.41% <0%> (-13.93%)	⬇️
cmd/common.go	5.71% <0%> (-8.58%)	⬇️
cmd/run.go	2.4% <0%> (-3.9%)	⬇️
cmd/archive.go	12.5% <0%> (-3.72%)	⬇️
cmd/inspect.go	9.67% <0%> (-3.15%)	⬇️
... and 17 more

---

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