[Proposal] Added support for change column in Schema #4745 #895

This will allow doing the following:
```
Schema::table('tests', function($table) {
    $table->string('name')->nullable()->change();
});
```
New function `compileChange` enables changing columns in MySQL grammar. Similar implementation can be easily added to sqlite, sqlserver and postgres.
I'm curious about these two lines. They create unexpected behavior IMO. For example, if I set a column as nullable, then later make a `change` migration that doesn't explicitly define a nullability setting for that column, I would expect it to retain is nullability; however, this line forces it back into a NOT NULL state. Seems like the expected behavior would be better reflected by just remove that line entirely.