[shell] Add `curlrc` file

When cURL is invoked, it always checks (unless -q is used) for a
default configuration file and uses it if found.

On UNIX-like systems, it will simply try to load `.curlrc` from
the determined home directory.

Providing the `~/.curlrc` file with some commonly used defaults
(e.g.: custom UA string, connection timeout time etc.), can really
ease and improve the experience when using cURL.

Reference: http://curl.haxx.se/docs/manpage.html#FILES