don't use ARGV in generator

## Case 1
```sh
rails generate active_admin:install --skip-users
```
give's:
```ruby
options = {"skip_namespace"=>false, "users"=>nil}
```
## Case 2
```sh
rails generate active_admin:install
```
give's:
```ruby
options = {"skip_namespace"=>false, "users"=>"devise"}
```
## Result
We can simply do this:
```ruby
options[:users]
```
insted of dealing with ARGV:
```ruby
if ARGV.include? "--skip-users"
```