Footer: include social link svgs as separate files instead of inlining (and add Tumblr, YouTube)

[maiamcc]

In standing up my own site, I added some social platforms to the footer, and then pulled out the svgs into individual files so I could reorder footer icons more easily; figured I'd make a PR against the theme repo in case this is useful to others!

(I work with SVGs by trial and error so the tumblr and youtube ones might be dumb, feel free to replace or remove entirely.)

[brianmaierjr]

Hey @maiamcc, thanks for the PR! I'm really sorry about how long it has taken me to see this....

I see the YouTube button has a black stroke on the icon and it doesn't match any of the buttons. Can we fix that and make it transparent to show the background color? I was thinking we probably want to document this in the README too since you added all these nice options. Would you add that to this PR as well?

[maiamcc]

Good catch, youtube icon fixed!

Happy to document these in the readme if you like, but for what it's worth, I think that including values for these in the config.yml would be plenty self-explanatory, e.g.:

title: Long Haul
description: Long Form Jekyll theme built with SASS
...
social:
    github: brianmaierjr
    twitter: brianmaierjr
    facebook: brianmaierjr
    email: brimaidesigns@gmail.com
    linkedin: brianmaierjr
    youtube: RickAstleyYT
    tumblr: i-am-a-fish

If you want to go that route, let me know what accounts I should include there (presumably yours and not just trolling ones); or lmk if you'd still prefer the readme and I can do that instead!

[brianmaierjr]

@maiamcc You're the best! You're right, we probably don't need to do anything in the README. Thank you!