Add support for 'ca' config file option

[vvmnnnkv]

Summary
npm supports specifying CA contents directly in the .npmrc file, using ca option.
yarn does not currently support this, as mentioned in several comments, e.g.:

• https://github.com/yarnpkg/website/issues/128#issue-182372920
• Yarn doesn't read from .npmrc Files for custom registry settings #606 (comment)

Test plan
Localhost is running Sinopia with cert signed by self-signed root CA.
Below is .npmrc file contents (root CA cert is removed for brevity):

registry=https://localhost:4873
ca[]="-----BEGIN CERTIFICATE----- ... -----END CERTIFICATE-----"

npm output:

$ npm install leftpad
leftpad@0.0.0 node_modules/leftpad

yarn output:

$ yarn add leftpad
yarn add v0.16.1
[1/4] Resolving packages...
error An unexpected error occured, please open a bug report with the information provided in "/home/yarn/bin/yarn-error.log".
info Visit https://yarnpkg.com/en/docs/cli/add for documentation about this command.

$ cat yarn-error.log 
...
  leftpad@^0.0.0:
    version "0.0.0"
    resolved "https://localhost:4873/leftpad/-/leftpad-0.0.0.tgz#020c9ad0787216ba0f30d79d479b4b355d7d39c3"

Trace: 
  Error: unable to verify the first certificate
  at Error (native)
  at TLSSocket.<anonymous> (_tls_wrap.js:1017:38)
  at emitNone (events.js:67:13)
  at TLSSocket.emit (events.js:166:7)
  at TLSSocket._init.ssl.onclienthello.ssl.oncertcb.TLSSocket._finishInit (_tls_wrap.js:582:8)
  at TLSWrap.ssl.onclienthello.ssl.oncertcb.ssl.onnewsession.ssl.onhandshakedone (_tls_wrap.js:424:38)

yarn output after the fix:

$ ./yarn add leftpad
yarn add v0.17.0-0
[1/4] Resolving packages...
[2/4] Fetching packages...
[3/4] Linking dependencies...
[4/4] Building fresh packages...
success Saved lockfile.
success Saved 1 new dependency.
└─ leftpad@0.0.0
Done in 0.77s.