Fix #2 - Add offline mode.

[jason-fox]

Online

• Copy Index JSON and Preview JSON to output.
• Use CDN for lunr.js

Offline

• Embed Index JSON and Preview JSON within lunr-client.js
• Amend <script> tag to refer to local lunr.js
• Add a local copy of lunr.js to output.

For Offline to work properly the custom plugin described in https://github.com/infotexture/dita-bootstrap/pull/95 must be present.

[jason-fox]

The fix involves using <xsl:merge> which is present in DITA-OT 3, but now mandatory for DITA-OT 4.1+

[infotexture]

OK, then I can set the required toolkit version to 3.7 (like dita-bootstrap) when we're ready to submit this to the registry.