Add the four settings the README documented but pixa lacked (closes #61)

access_control_allow_origin, upstream_fetch_timeout,
upstream_max_response_size and downstream_timeout were in the README
but unknown to pixa, so a config that followed it aborted startup. Each
is now a setting with its PIXA_ variable, defaulting to the value that
was fixed in the code: *, 30s, 50 MiB and 60s. Durations are positive Go
duration strings; the size is a positive whole number of bytes; the
origin is * or one scheme and host. An invalid value aborts startup
naming the key and value. downstream_timeout replaces HTTPWriteTimeout
for the server's write timeout and the per-request timeout.

Model: opus-5-5

config.example.yml

@@ -8,6 +8,10 @@
 # this file's env: section is set while the file loads, so it overrides
 # both the environment the process was started with and this file's own
 # key.
+#
+# Durations are Go duration strings such as 30s or 2m and must be
+# positive; a bare number has no unit and aborts startup. Sizes are a
+# whole number of bytes.
 
 # Server settings
 port: 8080
@@ -61,6 +65,21 @@ allow_http: false
 # Maximum concurrent connections per upstream host (default: 20)
 upstream_connections_per_host: 20
 
+# Time allowed for one fetch from an upstream host (default: 30s)
+upstream_fetch_timeout: 30s
+
+# Largest upstream response accepted, in bytes (default: 52428800, 50 MiB)
+upstream_max_response_size: 52428800
+
+# Time allowed for answering one client request, the upstream fetch
+# included, so keep it longer than upstream_fetch_timeout (default: 60s)
+downstream_timeout: 60s
+
+# The origin a browser lets read pixa's responses, sent as the CORS
+# Access-Control-Allow-Origin header: "*" (the default) is any site;
+# otherwise one origin, scheme and host only, such as https://example.com
+access_control_allow_origin: "*"
+
 # Maximum disk cache size in bytes. Explicit values are used exactly as
 # given; 0 disables the disk cache entirely (every request fetches and
 # processes uncached). When omitted, the default is 75% of the free