Prettier optional theme about vue-simple-calendar

[houseoftech]

I had already started on an alternate theme styled after macOS's Calendar app. See my demo-alternate-styling branch.

I started on this in order to figure out if any of the component markup needed to be reorganized to better support theming. I only came across one necessary change so far, the currentMonth button. There are some other things that could be added to support the theme a bit better, but they would require some involved changes, like supporting times.

Note: I'm switching the styles to SASS so you'll need to install a couple dependencies:

npm install node-sass --dev
npm install sass-loader --dev

[richardtallent]

I like it so far! If we can create themes that roughly mimic Google Calendar's new design, Outlook online, and Apple Calendar, I'd say we've achieved a pretty decently theme-able component.