Additional script that allows me to specify the feed URL manually

I'll first point you at a quick email between @julien51 and myself:

```
I'm trying to add subtome to a new site we're putting together and there's one page where I'd like to have a list of 'subscribe' buttons that each allow the user to subscribe to a different feed.

e.g. 

|-----------------------------------------|
|   Subscribe to news feed    |  http://example.com/news-feed.xml
|-----------------------------------------|

|-----------------------------------------|
|      Subscribe to  offers      |  http://offers.example.com/offers.xml
|-----------------------------------------| 
```

Quick, clear response:

```
Yes, it is perfeclt possible to have 2 different feeds (or more!) with different SubToMe buttons.

The 'trick' is to load the SubToMe iframe with differents params for each feeds.
So, for example, for the news feed you'll load:
https://subtome.com/#/subscribe?resource=<encoded url of the page of the resource, the news page for example>&feeds=http%3A%2F%2Fexample.com%2Fnews-feed.xml

And for the offers's page:
https://subtome.com/#/subscribe?resource=<encoded url of the page of the resource, the offers page for example>&feeds=http%3A%2F%2Fexample.com%2Foffers.xml

Also, make sure the iframe has the following CSS properties:
display:block; 


position:fixed; 
top:0px; 
left:0px; 
width:100%; 
height:100%; 
border:0px; 
background: transparent; 
z-index: 2147483647'

The rest is completely up to you in terms of customization. Feel free to check the "default" load.js https://www.subtome.com/load.js to see exactly how we load the iframe 'by default' (when there is only a single feed!).
```

Yeah, thing is I'm a lazy developer and really couldn't be bothered to do all of this. Ok, the fact that I'm making a PR contradicts that point, but I wanted it to be easier to achieve what I was after.

So what this pull request does is add an additional file very similar to `load.js` but isn't a self-calling anonymous function.  We load the `load-custom.js` script from subtome.com in the normal way and then call `subToMe()` in my `onclick` event.  This works just the same as the existing `load.js` script (i.e. it will scrape the page for RSS feeds). 

Now if I pass an argument, that argument becomes the feed URL so I can point my subscribe links at different feeds instead.

You may think this is unnecessary, but I thought I'd make the PR just in case you thought it was a useful addition. In the project I made this for I've included the script locally to get the same functionality.  The advantage of it being in the __subtome__ repo is that it gets the love of upgrades and bug fixes that everyone benefits from :)

Thanks, Lloyd.