Separate the "joinString" content from the concatenated files by line breaks

[prantlf]

The default value of joinString is a line break. If it is changed to something else, like to "---" (a full horizontal line), no line break will be appended. If the first concatenated file was not ended by a line break, the text "---" will be inserted at the end of the last line and will not be recognized as the markdown horizontal line. The leading line break is necessary to separate the joinString value from the first file. The trailing line break will separate the joinString value from the second file's content by an empty line.