home-manager: add --(no-)substitute options

Fixes #312

doc/man-home-manager.xml

@@ -126,6 +126,10 @@
     --show-trace
    </arg>
     
+   <arg>
+    --(no-)substitute
+   </arg>
+    
    <arg>
     <group choice="req"> 
     <arg choice="plain">
@@ -446,6 +450,18 @@
      </para>
     </listitem>
    </varlistentry>
+   <varlistentry>
+    <term>
+     <option>--(no-)substitute</option>
+    </term>
+    <listitem>
+     <para>
+      Passed on to <citerefentry>
+      <refentrytitle>nix-build</refentrytitle>
+      <manvolnum>1</manvolnum> </citerefentry>.
+     </para>
+    </listitem>
+   </varlistentry>
    <varlistentry>
     <term>
      <option>-v</option>

home-manager/home-manager

@@ -431,6 +431,7 @@ function doHelp() {
     echo "  --max-jobs NUM"
     echo "  --option NAME VALUE"
     echo "  --show-trace"
+    echo "  --(no-)substitute"
     echo
     echo "Commands"
     echo
@@ -507,7 +508,8 @@ while [[ $# -gt 0 ]]; do
             PASSTHROUGH_OPTS+=("$opt" "$1")
             shift
             ;;
-        --keep-failed|--keep-going|--show-trace)
+        --keep-failed|--keep-going|--show-trace\
+            |--substitute|--no-substitute)
             PASSTHROUGH_OPTS+=("$opt")
             ;;
         -v|--verbose)