remark-parse: fix whitespace only lines in paragraphs

In commonmark, paragraphs can be split by blank lines even if those
lines contain whitespace.

Currently, in remark, paragraphs will be split by blank lines only if
those lines contain fewer than four characters worth of whitespace,
because the code to detect following indented content checks for an
indentation but doesn't actually check for content.

This commit fixes this behaviour by making sure that there is content
following the indentation before absorbing the next paragraph.

Closes GH-348.
Reviewed-by: Christian Murphy <Christian.Murphy.42@gmail.com>
Reviewed-by: Titus Wormer <tituswormer@gmail.com>

packages/remark-parse/lib/tokenize/paragraph.js

@@ -62,7 +62,7 @@ function paragraph(eat, value, silent) {
         position++;
       }
 
-      if (size >= TAB_SIZE) {
+      if (size >= TAB_SIZE && character !== C_NEWLINE) {
         index = value.indexOf(C_NEWLINE, index + 1);
         continue;
       }

test/fixtures/input/paragraphs-and-indentation.text

@@ -6,6 +6,10 @@ This is a paragraph
 This is a paragraph
    and this is further text
 
+This is a paragraph
+
+and this is a new paragraph
+
 This is a paragraph with some asterisks
     ***