Expand sigil_CURL in Mix task

[kevinschweikert]

Closes #23

With mix curl_req.expand all the files in the lib directory are getting scanned for a sigil_CURL in their AST. I don't know what the preferred scope of files should be. This is currently a draft, because while it works with this code

def test do
  ~CURL(https://example.com)
end

it does not, because Sourceror.get_range(quoted) returns nil because somehow it cannot infer the range of the sigil when we do something like this:

def test do
  ~CURL(https://example.com) == true
end

If anybody has a solution, i would be very happy for some help or hints in the direction!