Add proxying support

[17320]

This pull request implements two new hexo-serve options that allow a user to proxy certain paths to a remote target. Useful for example to test API integrations when the API endpoints are called via relative URLs from the static part of the site.

Example

Assuming that you have your API deployment running locally on port 3001, the following command will proxy all URLs starting with /api to localhost:3001.

hexo serve -P /api -U http:localhost:3001/

Note: The proxied paths must not already exist in the static part of the site.

[coveralls]

Coverage decreased (-0.4%) to 94.118% when pulling c63f521 on 17320:add-proxy-support into de57a80 on hexojs:master.

[JLHwung]

Conventionally http://example.com is of documental usage.

Also for the sake of simplicity I suggest simply pass on the option to http-proxy-middleware directly so people can make most use of the potential.

The following suggestion also remove the proxyPath: undefined settings.

Suggested change

	proxyUrl: undefined
	httpProxy:
	context: /foo/bar
	config:
	target: http://example.com
	changeOrigin: true

Here the naming of httpProxy corresponds to http-proxy-middleware, context, config to the signature in the document here.