ShareDir: include simple styling function.

**Which issues (if any) does this change solve? (please link them here)**

No known reported issues, although I have considered making one.  In a more general sense, the issue is that with the recent design changes, Goodie developers keep writing their own CSS readers and appenders.  In addition to this being a lot of duplicated effort, it's quite easy to write one such that it will read the file on each answer.

**Briefly describe your fix. Are there any important details we should know about?**

At the time we are noting that a share directory exists, we also check to see if it has a top-level 'style.css'.  If so, it will read that file (once) and prepare a function to concatenate its contents with any provided arguments (presumably the HTML IA result.)

**Are you having any problems? Do you need our help with anything?**

While this will certainly work, I am not certain it is exactly the right solution in exactly the correct place.  If this PR (or similar) is accepted, I will do updates to the documentation (and perhaps a few Goodies) to enhance its value.

Also, in the POD I continued to use non-standard things which cause my parser to complain.  I presume there is some sort of preprocessing which makes it better.
@nilnilnil Do you mean automatically apply style.css to the `html` portion of the output?  I didn't do this for two reasons:

-That might be a somewhat surprising change at first merge.
- It seems unlikely, but I didn't know if some answers might use dynamically selected (or elided) CSS files.

If it's OK to do this, I will figure out the best way to accomplish that and make it so.