home-manager: support --option argument

Fixes #784

doc/man-home-manager.xml

@@ -102,6 +102,10 @@
      </group>
    </arg>
     
+   <arg>
+    --option <replaceable>name</replaceable> <replaceable>value</replaceable>
+   </arg>
+    
    <arg>
     --cores <replaceable>number</replaceable>
    </arg>
@@ -370,6 +374,18 @@
      </para>
     </listitem>
    </varlistentry>
+   <varlistentry>
+    <term>
+     <option>--option <replaceable>name</replaceable> <replaceable>value</replaceable></option>
+    </term>
+    <listitem>
+     <para>
+      Passed on to <citerefentry>
+      <refentrytitle>nix-build</refentrytitle>
+      <manvolnum>1</manvolnum> </citerefentry>.
+     </para>
+    </listitem>
+   </varlistentry>
    <varlistentry>
     <term>
      <option>--cores <replaceable>number</replaceable></option>

home-manager/home-manager

@@ -429,6 +429,7 @@ function doHelp() {
     echo "  --keep-failed"
     echo "  --keep-going"
     echo "  --max-jobs NUM"
+    echo "  --option NAME VALUE"
     echo "  --show-trace"
     echo
     echo "Commands"
@@ -498,6 +499,10 @@ while [[ $# -gt 0 ]]; do
         -n|--dry-run)
             export DRY_RUN=1
             ;;
+        --option)
+            PASSTHROUGH_OPTS+=("$opt" "$1" "$2")
+            shift 2
+            ;;
         --max-jobs|--cores)
             PASSTHROUGH_OPTS+=("$opt" "$1")
             shift