Allow hard-coded values to be set

Addresses the first part of issue #12 by making `autoclose`,
`keyboardNavigation`, and `multidate` settable properties.
Add demo documentation for changeable properties

Second part of issue #12, updated demo page for sl-date-picker to
document `autoclose`, `keyboardNavigation`, and `multidate` properties.
Add number as alternate `multidate` type
Merge pull request #101 from joshforisha/feature/date-picker-properties

Make date-picker properties configurable instead of hard-coded