fix(gitignore): ignore dist folder
feat(gulp): reload on server changes
fix(task-name): replace serve:dist by preview for clarity purpose

`preview` is easier to remind, and represents more the task purpose.
It also have the advantage to be less `generator-angular-fullstack`
oriented.

`preview` is just for testing that all is ok, so it not use the
livereload and nodemon abilities. Using `serve` term could cause
some confusions, let's bypass that.