Made date format configurable using parameter

[ilikeorangutans]

Closes #12

As discussed in issue #12 this makes the date format configurable via a param. I don't see a way of how a template could provide a default value for a param, from what I can tell the config.toml in a theme is just to provide an example.

Also, sorry about the trimmed whitespaces at the end of the line, my editor does that and I didn't realize until I put this up.

@aos what do you think?

[aos]

Hey @ilikeorangutans thanks for the PR! I've been very busy this last week so I'll look at it this weekend if I have time

[ilikeorangutans]

No worries, let me know what you think. Already building my own blog against my fork, so I'm good. Just thought I'd give back! 😄

[aos]

So I think the addition of a Params section for dateformat is great, but I think we can still do with a default one like so:

{{ if .Site.Params.dateformat }}
  {{ $page.Date.Format .Site.Params.dateformat }}
{{ else }}
  {{ $page.Date.Format "01-02-2006" }}
{{ end }}

It is unfortunately more verbose, but this is the cleanest way to go about it if we want a drop-in theme template without having to add anything extra on the config from the get-go.

[aos]

Also, I think the datetime attribute of <time /> doesn't necessarily need to be formatted and can just be given standard UTC time as its strictly for machine parsing or accessibility.

I forgot to change it when initially creating this template. If you want to work on that as well, that would be great!

[aos]

Hey @ilikeorangutans -- I've went ahead and implemented these changes. See #15 for details. I'll be closing your PR. Thanks for the input and help!