Added Vim segment plugin for the Asynchronous Linter Engine (ALE).

This segment has the same functionality and interface as the Syntastic
Vim segment plugin, and the code is largely adapted from it.
Documentation stub is also included

docs/source/configuration/segments/vim.rst

@@ -9,6 +9,12 @@ Vim segments
 Plugin-specific segments
 ========================
 
+Asynchronous Linter Engine (ALE) segments
+-----------------------------------------
+
+.. automodule:: powerline.segments.vim.plugin.ale
+   :members:
+
 Syntastic segments
 ------------------
 

powerline/segments/vim/plugin/ale.py

@@ -0,0 +1,97 @@
+# vim:fileencoding=utf-8:noet
+"""
+Asynchronous Lint Engine (ALE) plugin
+
+Largely adapted from Sytastic plugin
+"""
+
+from __future__ import (
+	unicode_literals, division, absolute_import, print_function
+)
+try:
+	import vim
+except ImportError:
+	vim = object()
+from powerline.bindings.vim import vim_global_exists
+from powerline.theme import requires_segment_info
+
+
+# To turn on logging, in the Powerline configuration fie config.json,
+# in the "common" key, add log_file and log_level key-value pairs as
+# appropriate and desired, for example:
+#
+# "common": {
+#	  "term_truecolor": false,
+#	  "log_file": "/mylogdir/powerline.log",
+#	  "log_level": "INFO"
+# },
+#
+# and then turn on debug internal variable to True
+@requires_segment_info
+def ale(
+	segment_info,
+	pl,
+	err_format='ERR:  {first_line} ({num}) ',
+	warn_format='WARN:  {first_line} ({num}) '
+):
+	"""
+	Show whether Asynchronous Lint Engine (ALE) has found any errors
+	or warnings
+
+	:param str err_format:
+		Format string for errors.
+
+	:param str warn_format:
+		Format string for warnings.
+
+	Highlight groups used: ``ale:warning`` or
+	``warning``, ``ale:error`` or ``error``.
+	"""
+	# pylint: disable=C0103,W0613
+	debug = False
+	bufnr = str(segment_info['bufnr'])
+	ale_enabled_exists = vim_global_exists('ale_enabled')
+	ale_enabled = ale_enabled_exists and int(vim.eval('g:ale_enabled'))
+	if debug:
+		pl.info('ale_enabled_exists: {0}'.format(ale_enabled_exists))
+		pl.info('ale_enabled: {0}'.format(ale_enabled))
+	if not ale_enabled:
+		return None
+	cmd = 'ale#statusline#Count({0}).total'.format(bufnr)
+	if debug:
+		pl.info('cmd: {0}'.format(cmd))
+	has_errors = int(vim.eval(cmd))
+	if debug:
+		pl.info('has_errors: {0}'.format(has_errors))
+	if not has_errors:
+		return
+	cmd = 'ale#engine#GetLoclist({0})'.format(bufnr)
+	if debug:
+		pl.info('cmd: {0}'.format(cmd))
+	issues = vim.eval(cmd)
+	errors, warnings = [], []
+	for issue in issues:
+		if issue['type'] == 'E':
+			errors.append(issue)
+		elif issue['type'] == 'W':
+			warnings.append(issue)
+	segments = []
+	if errors:
+		segments.append(
+			{
+				'contents': err_format.format(
+					first_line=errors[0]['lnum'], num=len(errors)
+				),
+				'highlight_groups': ['ale:error', 'error'],
+			}
+		)
+	if warnings:
+		segments.append(
+			{
+				'contents': warn_format.format(
+					first_line=warnings[0]['lnum'], num=len(warnings)
+				),
+				'highlight_groups': ['ale:warning', 'warning'],
+			}
+		)
+	return segments