Add ability to import modules with dashes or spaces

Any module with underscores in it will now attempt to look for 3 files,
first the original, then with dashes, then with spaces, ie:
import addressbook_v_2_capnp
will search for 'addressbook_v_2.capnp', 'addressbook-v-2.capnp', and
'addressbook v 2.capnp'.

test/addressbook-with-dashes.capnp

@@ -0,0 +1,39 @@
+@0xd33206731939e03b;
+
+const qux :UInt32 = 123;
+
+struct Person {
+  id @0 :UInt32;
+  name @1 :Text;
+  email @2 :Text;
+  phones @3 :List(PhoneNumber);
+
+  struct PhoneNumber {
+    number @0 :Text;
+    type @1 :Type;
+
+    enum Type {
+      mobile @0;
+      home @1;
+      work @2;
+    }
+  }
+
+  employment :union {
+    unemployed @4 :Void;
+    employer @5 :Employer;
+    school @6 :Text;
+    selfEmployed @7 :Void;
+    # We assume that a person is only one of these.
+  }
+}
+
+struct Employer {
+  name @0 :Text;
+  boss @1 :Person;
+}
+
+struct AddressBook {
+  people @0 :List(Person);
+}
+