Add default options when defining job_type

Currently there is no way to define job_type with optional options, we will have to define exact every option in template, this cause some issues: https://github.com/javan/whenever/commit/61275d7e076f1a488e74fb588cf226f84eb5ba76#commitcomment-3677817

For example:

```schedule.rb
job_type :foo, "script :arguments"

every 1.minute do
  foo "bar"
  foo "baz" arguments: ""
end
```

This generates cronlines as below:
```
* * * * * bash -l -c 'script bar :arguments' # BAD
* * * * * bash -l -c 'script baz' # What we want
```

This PR aims to provide extra feature to define default options that can help us preset options that are **optional**, to avoid wrong cronline generation.

```schedule.rb
job_type :foo, "script :arguments", arguments: ""
...
```