Minor refactoring. Move the definition of <code>BLOB_APPEND_BLOB()</code>
macro from <code>markdown_html.c</code> to <code>blob.c</code> so that
it could be used outside of <code>markdown_html.c</code>.
Also rename it to <code>blob_appendb()</code> for consistency with
<code>blob_appendf()</code> and other API.
Within <code>markdown.c</code> use that newly available macro where appropriate.
Within <code>markdown_html.c</code> use it for footnotes-relevant code.
Other invocations of <code>BLOB_APPEND_BLOB()</code> within
<code>markdown_html.c</code> are left intact (they use an alias) in order to
simplify the potential merge with the trunk.