add conf support for default cwd and serial

read config from '~/.ansible/ansible.ini' by default,  and can relocated using -F
sample config:
> [ansible-shell]
> cwd = ~webserver
> serial = 20

then ansible-shell will use these as default value on startup:
```shell
[chengshengbo@lg-mu-cs00 ~]$ ansible-shell
Welcome to the ansible-shell.
Type help or ? to list commands.

chengshengbo@~webserver (188)[s:20]$
```