Make feh respect CURL_CA_BUNDLE similar to the `curl` tool.

diff --git a/src/imlib.c b/src/imlib.c
index dfb79aa..82a9865 100644
--- a/src/imlib.c
+++ b/src/imlib.c
@@ -429,6 +429,10 @@ static char *feh_http_load_image(char *url)
 			if (opt.insecure_ssl) {
 				curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, 0);
 				curl_easy_setopt(curl, CURLOPT_SSL_VERIFYHOST, 0);
+			} else {
+				// Allow the user to specify custom CA certificates.
+				curl_easy_setopt(curl, CURLOPT_CAINFO,
+						getenv("CURL_CA_BUNDLE"));
 			}
 
 			res = curl_easy_perform(curl);