I came across this really useful snippet of code from http://www.bergspot.com/blog/2012/05/formatting-xml-in-sublime-text-2-xmllint/ to format XML data using the xmllint command. I've modified it to iron out bugs I had using it with Sublime Text 3 (Build 3047). To add it to Sublime, place this tidy_xml_lint.py file in your Sublime install's Pack…

Main.sublime-menu

[
    {
        "caption": "Selection",
        "children": [
            {
                "caption": "Format",
                "children": [
                    {
                        "caption": "Tidy with XML Lint",
                        "command": "tidy_xml_lint"
                    }
                ],
                "id": "format"
            }
        ],
        "id": "selection"
    }
]

tidy_xml_lint.py

import sublime
import sublime_plugin
import subprocess

class TidyXmlLintCommand(sublime_plugin.TextCommand):
    def run(self, edit):
        command = "XMLLINT_INDENT='\t' xmllint --format --encode utf-8 -"

        # help from http://www.sublimetext.com/forum/viewtopic.php?f=2&p=12451
        xmlRegion = sublime.Region(0, self.view.size())
        p = subprocess.Popen(command, bufsize=-1, stdout=subprocess.PIPE, stderr=subprocess.PIPE, stdin=subprocess.PIPE, shell=True)
        result, err = p.communicate(self.view.substr(self.view.sel()[0]).encode('utf-8'))

        err = err.decode('utf-8')

        if err != "":
            print("tidy_xml_lint: Error:" + err)
            self.view.set_status('xmllint', "xmllint: " + err)
            sublime.set_timeout(self.clear,10000)
        else:
            self.view.replace(edit, self.view.sel()[0], result.decode('utf-8'))
            sublime.set_timeout(self.clear,0)

    def clear(self):
        self.view.erase_status('xmllint')