Added -variant to disable php feature.

Implemented -variant for removing variant.

Ex1. build php with default variant and sqlite support but without pdo and pdo-sqlite driver
```
phpbrew install php-5.3.21 +default+sqlite-pdo
```
Ex2. build php enable all features but without pdo and sqlite
```
phpbrew install php-5.3.21 +all-sqlite-pdo
```

Ref: #32

Signed-off-by: Rack Lin <racklin@gmail.com>