Generate relative paths in JS source maps

Also, add CLI options to manipulate prefixes of path

See KT-4078

js/js.frontend/src/org/jetbrains/kotlin/js/config/JSConfigurationKeys.java

@@ -29,6 +29,12 @@ public class JSConfigurationKeys {
     public static final CompilerConfigurationKey<Boolean> SOURCE_MAP =
             CompilerConfigurationKey.create("generate source map");
 
+    public static final CompilerConfigurationKey<String> SOURCE_MAP_PREFIX =
+            CompilerConfigurationKey.create("prefix to add to paths in source map");
+
+    public static final CompilerConfigurationKey<List<String>> SOURCE_MAP_SOURCE_ROOTS =
+            CompilerConfigurationKey.create("base directories used to calculate relative paths for source map");
+
     public static final CompilerConfigurationKey<Boolean> META_INFO =
             CompilerConfigurationKey.create("generate .meta.js and .kjsm files");
 

js/js.frontend/src/org/jetbrains/kotlin/js/config/JsConfig.java

@@ -103,6 +103,16 @@ public class JsConfig {
         return getConfiguration().getList(JSConfigurationKeys.LIBRARIES);
     }
 
+    @NotNull
+    public String getSourceMapPrefix() {
+        return configuration.get(JSConfigurationKeys.SOURCE_MAP_PREFIX, "");
+    }
+
+    @NotNull
+    public List<String> getSourceMapRoots() {
+        return configuration.get(JSConfigurationKeys.SOURCE_MAP_SOURCE_ROOTS, Collections.singletonList("."));
+    }
+
     @NotNull
     public List<String> getFriends() {
         if (getConfiguration().getBoolean(JSConfigurationKeys.FRIEND_PATHS_DISABLED)) return Collections.emptyList();