Fix DOM mount API, merge &. mount

Address issues with the DOM mount API, clarify merge vs. mount.

Breaking Changes

• Removed treetop.updateElement from the JS API, (replaced with the following)
• Add treetop.mount static method for explicitly replacing an element
• Add treetop.merge static method for applying an new template, respecting treetop-merge attributes

Fix

• Detect recursive merges and throw an error

Changes

• Add an .editorconfig and apply an automatic code formatter
• Remove internal defaultComposition method
• Minor improvements to API reference markdown

Adopt changes to Treetop protocol in v0.3.0 of the Go library.

Changed

• Treetop content type changed to application/x.treetop-html-template+xml
• Response URL is renamed to X-Page-URL and is now optional
• Response body can now come wrapped in a <template>...</template> tag

Redirects

• Redirects now use Location header
• Status will continue to be 200
• There must be a header named X-Treetop-Redirect with a value of "SeeOther"

First coherent version of the API somewhat stabilized

v0.5.4

Bugfix: JS exception when getAttribute returns full for input type