Make the is_number regex accept numbers beginning with -0

Also ends up simpler, too - originally I was trying to avoid accepting
e.g. "+0" or "-0" but that's not really a big deal.

Fixes #14

debian/dump1090-mutability.config

@@ -133,7 +133,7 @@ is_ipaddrish_or_empty() {
 }
 
 is_number() {
-  if echo "$1" | grep -Eq '^(0|[+-]?[1-9][0-9]*)(\.[0-9]+)?$'; then return 0; else return 1; fi
+  if echo "$1" | grep -Eq '^([+-]?[0-9][0-9]*)(\.[0-9]+)?$'; then return 0; else return 1; fi
 }
 
 is_number_or_empty() {