support for ~ (home dir) in cafile (and other directives accepting a file path)

[kevinv11n]

Do you want to request a feature or report a bug?
Bug

What is the current behavior?
~ is not properly evaluated when used in the cafile directive in .npmrc

If the current behavior is a bug, please provide the steps to reproduce.

Consider the scenario of a private npm registry that uses a self-signed SSL certificate. A user's ~/.npmrc would look like this:

registry=https://internal-registry/...
cafile=~/custom-certs.pem

Running yarn install results in this error message: error Could not open cafile: ENOENT: no such file or directory, open '~/custom-certs.pem'

If you change the cafile directive to cafile=/users/kevin/custom-certs.pem then yarn install works fine.

What is the expected behavior?
Use of ~ in cafile should expand to the user's home directory. It should do the same expansion in other directives that accept file paths.

Please mention your node.js, yarn and operating system version.
node: v6.2.0
yarn: v0.21.3