Add an option for emojis at line ends

Some people want to render emojis at line ends, even
though they don't have a whitespace before:(

Others don't want such a `*feature:*`
In the line above the  `:*` would be a kissing smiley instead of the
normal text that I wanted to see.

This commit adds the option `sloppier_emoticons_at_line_end` which
enables emoticons at line ends even if they have no whitespace before.
This option is set to `true` by default to be consistent with the old
behaviour.

Merging this would fix a bug at my side project: https://github.com/tessi/openproject-emoji/issues/5