Avoid cutoff rss in posts with tables

[javierluraschi]

Blog posts with tables that contains tables, e.g.:

    +-------------------+-----+---------+
    |               time|value|value_sum|
    +-------------------+-----+---------+
    |1970-01-01 00:00:01|    1|      1.0|
    |1970-01-01 00:00:02|    4|      5.0|
    |1970-01-01 00:00:03|    9|     14.0|
    |1970-01-01 00:00:04|   16|     29.0|
    +-------------------+-----+---------+

would render the RSS cutoff since the current regular expression eagerly matches entries until an arbitrary --- entry. Instead, we find all entries of --- and remove from start to end of header using stringr.