Only shorten feeds at occurrences of delimiters

[wiwie]

With the content limit (default: 140) it can happen, that words are cut in the middle. This option allows to specify a list of delimiters which are used when shortening post contents:

• We only cut at occurrences of the specified delimiters;
• We probe the delimiters in order, and use the first one for shortening that occurs in the post content;
• If none of them occurs, we cut at content_limit (e.g. 140);

[coveralls]

Coverage decreased (-21.9%) to 78.125% when pulling b973425 on wiwie:post-content-delimiter into 19ab397 on hexojs:master.

[coveralls]

Coverage decreased (-33.3%) to 66.667% when pulling 57c2dd0 on wiwie:post-content-delimiter into 19ab397 on hexojs:master.

[coveralls]

Coverage decreased (-33.3%) to 66.667% when pulling 57c2dd0 on wiwie:post-content-delimiter into 19ab397 on hexojs:master.

[coveralls]

Coverage decreased (-33.3%) to 66.667% when pulling bb06418 on wiwie:post-content-delimiter into 19ab397 on hexojs:master.

[tomap]

Your PR looks interesting but contains too many changes to be merged. could you redo your PR with a single change at the time?

[tomap]

I believe most of this PR is already in master. Maybe not the thumbnail part? would you be willing to make a new PR?