Enable SSI on Apache for a directory tree

In the 000-default.conf:

<Directory /path/to/parent/directory>
  Options +Includes
  AddType text/html .html
# Either of the next two lines should work by itself, un-comment as necessary
  AddOutputFilter INCLUDES .html
  #AddHandler server-parsed .html
</Directory>

Once the above is set, restart apache2 and you can use directives of the sort:

<!--#include virtual="path/from/the/containing/included_file.anyext" -->