Fix Generate::migration()

This fixes the root of https://github.com/fuel/oil/pull/177

Current magic migrations does not care of the case where the column name (or table name) contains underscores (or reserved words for example _to_, _from_ and _in_). 

In order to split the strict sense of the word, should not use underscores as concatenation character.
I propose the "." instead of  the "_".
But generating still allows "_" for compatibility.

The cases of current.
```sh
php oil g migration delete_is_visible_from_articles is_visible:bool
# The code generated doesn't specify table name.

php oil g migration add_to_mail_to_contact_to_users to_mail:varchar[255]
# The code generated doesn't specify table name.
```

This change enables the followings.
```sh
php oil g migration delete_is_visible_from_articles is_visible:bool
# The code generated is correct.

php oil g migration add.to_mail.to.contact_to_users to_mail:varchar[255]
# "." can be used to split the meaning
```