Support for setext-style (underlined) headers

Adds <tab> expansion of - and = lines to match the preceding header
length. Also a command to fix the lengths of all headers at once.

Default.sublime-commands

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+[
+    {
+        "caption": "Fix Underlined Markdown Headers",
+        "command": "fix_all_underlined_headers"
+    }
+]

Default.sublime-keymap

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+[
+
+	{ "keys": ["tab"], "command": "complete_underlined_header", "context":
+		[
+			{ "key": "selector", "operator": "equal", "operand": "text.html.markdown" },
+			{ "key": "selection_empty", "operator": "equal", "operand": true, "match_all": true },
+			{ "key": "auto_complete_visible", "operator": "equal", "operand": false, "match_all": true },
+			{ "key": "preceding_text", "operator": "regex_contains", "operand": "^\\-+|^\\=+", "match_all": true }
+		]
+	}
+]

underlined_headers.py

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+"""Commands for working with with setext-style (underlined) Markdown headers.
+
+Header dashes can be completed with <tab>. For example:
+
+	This is an H2
+	-<tab>
+
+Becomes:
+
+	This is an H2
+	-------------
+
+Inspired by the similar TextMate command.
+
+Also adds "Fix Underlined Markdown Headers" to Tools > Command Palette. After modifying
+header text, this command will re-align the underline dashes with the new text length.
+
+"""
+import sublime, sublime_plugin
+import re, itertools
+
+# A run of ---- or ==== underline characters.
+SETEXT_DASHES_RE = re.compile( r'[-=]+', re.X )
+
+def fix_dashes(view, edit, text_region, dash_region):
+	"""Replaces the underlined "dash" region of a setext header with a run of
+	dashes or equal-signs that match the length of the header text."""
+
+	old_dashes = view.substr(dash_region)
+	first_dash = old_dashes[0]
+	new_dashes = first_dash * text_region.size()
+	view.replace(edit, dash_region, new_dashes)
+
+
+class CompleteUnderlinedHeaderCommand(sublime_plugin.TextCommand):
+	"""If the current selection is looks like a setext underline of - or = ,
+	then inserts enough dash characters to match the length of the previous
+	(header text) line."""
+
+	def run(self, edit):
+		for region in self.view.sel():
+			dashes_line = self.view.line(region)
+			# Ignore first list
+			if dashes_line.begin() == 0: continue
+
+			text_line = self.view.line(dashes_line.begin() - 1)
+			if text_line.begin() < 0: continue
+
+			dashes = self.view.substr(dashes_line)
+			m = SETEXT_DASHES_RE.match(dashes)
+			if m:
+				fix_dashes(self.view, edit, text_line, dashes_line)
+
+
+class FixAllUnderlinedHeadersCommand(sublime_plugin.TextCommand):
+	"""Searches for all setext headings resize them to match the preceding
+	header text."""
+
+	def description(self):
+		# Used as the name for Undo.
+		return 'Fix Underlined Markdown Headers'
+
+	def run(self, edit):
+		lines = self.view.split_by_newlines(sublime.Region(0, self.view.size()))
+		if len(lines) < 2: return
+
+		# Since we're modifying the text, we are shifting all the following
+		# regions. To avoid this, just go backwards.
+		lines = reversed(lines)
+
+		# Duplicate the iterator and next() it once to get farther ahead.
+		# Since lines are reversed, this will always point to the line *above*
+		# the current one: the text of the header.
+		prev_lines, lines = itertools.tee(lines)
+		next(prev_lines)
+
+		for text_line, dashes_line in itertools.izip(prev_lines, lines):
+			dashes_text = self.view.substr(dashes_line)
+			m = SETEXT_DASHES_RE.match(dashes_text)
+			if m:
+				fix_dashes(self.view, edit, text_line, dashes_line)