Introduce aliases functionality

Aliases can now be defined in 'aliases' section of topydo config file.

Examples:

[aliases]
showall = ls -x
purge = del -f
ical = ls -f ical

topydo.conf

@@ -46,3 +46,6 @@ append_parent_contexts      = 0
 ; context_color = magenta
 ; metadata_color = green
 ; link_color = light-cyan
+
+[aliases]
+;showall = ls -x

topydo/Commands.py

@@ -53,7 +53,6 @@ _SUBCOMMAND_MAP = {
     'tag': 'TagCommand',
 }
 
-
 def get_subcommand(p_args):
     """
     Retrieves the to-be executed Command and returns a tuple (Command, args).
@@ -82,11 +81,19 @@ def get_subcommand(p_args):
 
     result = None
     args = p_args
+    alias_map = config().aliases()
 
     try:
         subcommand = p_args[0]
 
-        if subcommand in _SUBCOMMAND_MAP:
+        if subcommand in alias_map:
+            real_subcommand, alias_args = alias_map[subcommand]
+            try:
+                result = import_subcommand(real_subcommand)
+                args = alias_args + args[1:]
+            except ImportError:
+                pass
+        elif subcommand in _SUBCOMMAND_MAP:
             result = import_subcommand(subcommand)
             args = args[1:]
         elif subcommand == 'help':

topydo/lib/Config.py

@@ -42,6 +42,7 @@ class _Config:
         """
         self.sections = [
             'add',
+            'aliases',
             'colorscheme',
             'dep',
             'ls',
@@ -279,6 +280,22 @@ class _Config:
         except ValueError:
             return self.defaults['add']['auto_creation_date'] == '1'
 
+    def aliases(self):
+        """
+        Returns dict with aliases names as keys and pairs of actual
+        subcommand and alias args as values.
+        """
+        aliases = self.cp.items('aliases')
+        alias_dict = dict()
+
+        for alias, meaning in aliases:
+            meaning = meaning.split()
+            real_subcommand = meaning[0]
+            alias_args = meaning[1:]
+            alias_dict[alias] = (real_subcommand, alias_args)
+
+        return alias_dict
+
 def config(p_path=None, p_overrides=None):
     """
     Retrieve the config instance.