Add platform-specific open and edit commands at init

The main motivation is to provide sensible default to make the
commnand "just work" without needing to fiddle with the configuration

pubs/commands/init_cmd.py

@@ -41,6 +41,8 @@ def command(conf, args):
 
     conf['main']['pubsdir'] = pubsdir
     conf['main']['docsdir'] = docsdir
+    conf['main']['open_cmd'] = config.default_open_cmd()
+    conf['main']['edit_cmd'] = config.default_edit_cmd()
     config.save_conf(conf)
 
     Repository(conf, create=True)

pubs/config/__init__.py

@@ -1 +1,2 @@
 from .conf import get_confpath, load_default_conf, load_conf, save_conf, check_conf
+from .conf import default_open_cmd, default_edit_cmd

pubs/config/conf.py

@@ -1,4 +1,6 @@
 import os
+import platform
+import shutil
 
 import configobj
 import validate
@@ -54,3 +56,37 @@ def save_conf(conf, path=None):
         path = get_confpath(verify=False)
     with open(path, 'wb') as f:
         conf.write(outfile=f)
+
+def default_open_cmd():
+    """Chooses the default command to open documents"""
+    if platform.system() == 'Darwin':
+        return 'open'
+    elif platform.system() == 'Linux':
+        return 'xdg-open'
+    elif platform.system() == 'Windows':
+        return 'start'
+    else:
+        return None
+
+def which(cmd):
+    try:
+        return shutil.which(cmd) # available in python 3.3
+    except AttributeError:
+        for path in ['.'] + os.environ["PATH"].split(os.pathsep):
+            filepath = os.path.join(path.strip('"'), cmd)
+            if os.path.isfile(path) and os.access(path, os.X_OK):
+                return filepath
+        return None
+
+def default_edit_cmd():
+    """Find an available editor"""
+    if 'EDITOR' in os.environ:
+        return os.environ['EDITOR']
+    elif platform.system() == 'Darwin' or 'Linux':
+        for editor in ['vim', 'nano', 'emacs', 'vi']:
+            if which(editor) is not None:
+                return editor
+    elif platform.system() == 'Windows':
+        return 'Notepad.exe | Out-Null' # wait for notepad to close
+    else:
+        return None