add support for specifying endpoint via environment + add docs

Fixes #38

Author: Stephen Gutekanst

README.md

@@ -58,13 +58,13 @@ Consult `src -h` and `src api -h` for usage information.
 Some Sourcegraph instances will be configured to require authentication. You can do so via the environment:
 
 ```sh
-SRC_ACCESS_TOKEN="secret" src ...
+SRC_ENDPOINT=https://sourcegraph.example.com SRC_ACCESS_TOKEN="secret" src ...
 ```
 
 Or via the configuration file (`~/src-config.json`):
 
 ```sh
-	{"accessToken": "secret"}
+	{"accessToken": "secret", "endpoint": "https://sourcegraph.example.com"}
 ```
 
 See `src -h` for more information on specifying access tokens.

cmd/src/main.go

@@ -91,6 +91,11 @@ func readConfig() (*config, error) {
 	if *endpoint != "" {
 		cfg.Endpoint = strings.TrimSuffix(*endpoint, "/")
 	}
+	if cfg.Endpoint == "" {
+		if endpoint := os.Getenv("SRC_ENDPOINT"); endpoint != "" {
+			cfg.Endpoint = endpoint
+		}
+	}
 	if cfg.Endpoint == "" {
 		cfg.Endpoint = "https://sourcegraph.com"
 	}