Added str_remove() and str_remove_all()

[Shians]

This is just an alias for str_replace(string, pattern, "") but I feel like it's a common enough usage to deserve its own functions.

[hadley]

Can you please add a bullet to NEWS? It should briefly describe the change (starting with name of the function), and crediting yourself with (@yourname, #issuenumber).

Thanks!

[Shians]

Thanks Hadley, I've updated the NEWS.

[hadley]

Thanks!