Added default command to synopsis.
Titles with multiple names for arguments.

This change adds the posibility to add multiple names for arguments,
e.g.  in a `git remote add` command the arguments can be `name` and
`url`, therefore in the help message it's going to render `<name>` and
`<url>` as separate arguments. A further improvement would be to handle
tpyes and arity for arguments too.
Made a couple of improvements in the help system and the rendering logic.

The changes include:
- showing which is the default command in a command group;
- one can specify multiple separated names for command arguments;
- replaced 'Whith no arguments' by 'By default' in command group message.

Merge branch 'feature/helpText' into develop
bumped version to 0.7.2