Create theme loading class

This class loads all segments from the theme, initializes the highlight
group, assigns all necessary properties based on the JSON theme
configuration, etc. By doing this we basically move the mksegment()
functionality into the theme loader, so this function can be removed at
the cost of making it more complicated to use Powerline without its
theme functionality.

powerline/theme.py

@@ -1,6 +1,44 @@
 # -*- coding: utf-8 -*-
 
+import importlib
+
 
 class Theme(object):
 	def __init__(self, ext, theme):
-		pass
+		self.segments = []
+
+		for side in ['left', 'right']:
+			for segment in theme['segments'].get(side, []):
+				contents = None
+				segment_type = segment.get('type', 'function')
+
+				if segment_type == 'function':
+					# Import segment function and assign it to the contents
+					function_module = 'powerline.ext.{0}.segments'.format(ext)
+					function_name = segment['name']
+					contents = getattr(importlib.import_module(function_module), function_name)
+				elif segment_type == 'string':
+					contents = segment.get('contents')
+				elif segment_type == 'filler':
+					pass
+				else:
+					raise TypeError('Unknown segment type: {0}'.format(segment_type))
+
+				self.segments.append({
+					'type': segment_type,
+					'highlight': segment.get('highlight', segment.get('name')),
+					'before': segment.get('before'),
+					'after': segment.get('after'),
+					'contents': contents,
+					'args': segment.get('args', {}),
+					'ljust': segment.get('ljust', False),
+					'rjust': segment.get('rjust', False),
+					'priority': segment.get('priority', -1),
+					'draw_divider': segment.get('draw_divider', True),
+					'side': side,
+					'exclude_modes': segment.get('exclude_modes', []),
+					'include_modes': segment.get('include_modes', []),
+				})
+
+	def get_segments(self):
+		return self.segments