NBStepper isLinear

[francisedulab]

Issue type

I'm submitting a ... (check one with "x")

• bug report
• [x ] feature request

Issue description

Current behavior:
There is no option for linear as shown in
https://material.angular.io/components/stepper/overview
the user can click on the step number and he will land on that step

Expected behavior:
when there is step 1,2,3 the user when clicks on step three directly should not be able to go there
it should be based on conditions

Steps to reproduce:
https://akveo.github.io/nebular/docs/components/stepper/overview#nbstepcomponent (use the validation)

Related code:
https://material.angular.io/components/stepper/examples (see enable linear mode)