Allow for specifying custom base url in install process

[srubin]

Allows for setting FFMPEG_FFPROBE_STATIC_BASE_URL environment variable, and will fetch the ffmpeg/ffprobe binaries from that location.

Example:

FFMPEG_FFPROBE_STATIC_BASE_URL="https://www.google.com" yarn install
yarn install v1.17.3
[1/4] 🔍  Resolving packages...
[2/4] 🚚  Fetching packages...
[3/4] 🔗  Linking dependencies...
[4/4] 🔨  Building fresh packages...
error /Users/srubin/code/test/node_modules/ffmpeg-ffprobe-static: Command failed.
Exit code: 1
Command: node install.js
Arguments:
Directory: /Users/srubin/code/test/node_modules/ffmpeg-ffprobe-static
Output:
[ffmpeg-ffprobe-static] Using custom base url: https://www.google.com
Error: Download failed.
    at /Users/srubin/code/test/node_modules/ffmpeg-ffprobe-static/install.js:94:20
    at /Users/srubin/code/test/node_modules/ffmpeg-ffprobe-static/node_modules/@derhuerst/http-basic/lib/index.js:117:20
    at /Users/srubin/code/test/node_modules/ffmpeg-ffprobe-static/node_modules/@derhuerst/http-basic/lib/index.js:173:24
    at /Users/srubin/code/test/node_modules/ffmpeg-ffprobe-static/node_modules/@derhuerst/http-basic/lib/index.js:272:28
    at /Users/srubin/code/test/node_modules/ffmpeg-ffprobe-static/node_modules/@derhuerst/http-basic/lib/index.js:302:17
    at ClientRequest.<anonymous> (/Users/srubin/code/test/node_modules/ffmpeg-ffprobe-static/node_modules/@derhuerst/http-basic/lib/index.js:337:13)
    at Object.onceWrapper (events.js:422:26)
    at ClientRequest.emit (events.js:315:20)
    at HTTPParser.parserOnIncomingClient [as onIncoming] (_http_client.js:641:27)
    at HTTPParser.parserOnHeadersComplete (_http_common.js:126:17) {
  url: 'https://www.google.com/b4.3.1-rc.5/ffmpeg-darwin-x64',
  statusCode: 404
}
info Visit https://yarnpkg.com/en/docs/cli/install for documentation about this command.

[srubin]

Goal of this is to be able to vendor builds of ffmpeg/ffprobe that we don't want to upload publicly.