configure using git config as well as the command line or git aliases

Although it is possible to use a git aliases ```git config --global alias.ansical 'cal --ansi'``` to persistently configure behaviour of git-cal, this PR adds the ability to store the settings in a "calendar" stanza. Setting via the command line thus: ```git config --global calendar.format ansi```

The full list in a ```~/.gitconfig``` might look like.
```ini
[calendar]
    format = ansi
    period = 5
    author = karthik
```
I look forward to your thoughts on this.