Generate relative paths in JS source maps

Also, add CLI options to manipulate prefixes of path

See KT-4078

compiler/testData/cli/js/sourceMapPrefix.args

@@ -0,0 +1,7 @@
+$TESTDATA_DIR$/sourceMap.kt
+-no-stdlib
+-source-map
+-source-map-prefix
+http://localhost:8080/src/
+-output
+$TEMP_DIR$/out.js