Add the ability to prepend to the proxy path

This is for applications like Jupyter that aren't base path agnostic.

src/node/app/proxy.ts

@@ -24,7 +24,7 @@ export class ProxyHttpProvider extends HttpProvider {
     const port = route.base.replace(/^\//, "")
     return {
       proxy: {
-        base: `${route.providerBase}/${port}`,
+        strip: `${route.providerBase}/${port}`,
         port,
       },
     }
@@ -35,7 +35,7 @@ export class ProxyHttpProvider extends HttpProvider {
     const port = route.base.replace(/^\//, "")
     return {
       proxy: {
-        base: `${route.providerBase}/${port}`,
+        strip: `${route.providerBase}/${port}`,
         port,
       },
     }

src/node/http.ts

@@ -36,9 +36,13 @@ export type Query = { [key: string]: string | string[] | undefined }
 
 export interface ProxyOptions {
   /**
-   * A base path to strip from from the request before proxying if necessary.
+   * A path to strip from from the beginning of the request before proxying
    */
-  base?: string
+  strip?: string
+  /**
+   * A path to add to the beginning of the request before proxying.
+   */
+  prepend?: string
   /**
    * The port to proxy.
    */
@@ -826,10 +830,11 @@ export class HttpServer {
     // sure how best to get this information to the `proxyRes` event handler.
     // For now I'm sticking it on the request object which is passed through to
     // the event.
-    ;(request as ProxyRequest).base = options.base
+    ;(request as ProxyRequest).base = options.strip
 
     const isHttp = response instanceof http.ServerResponse
-    const path = options.base ? route.fullPath.replace(options.base, "") : route.fullPath
+    const base = options.strip ? route.fullPath.replace(options.strip, "") : route.fullPath
+    const path = normalize("/" + (options.prepend || "") + "/" + base, true)
     const proxyOptions: proxy.ServerOptions = {
       changeOrigin: true,
       ignorePath: true,