add options to set context options

implement the following functions:

- zctx_set_iothreads/2,
- zctx_set_linger/2,
- zctx_set_pipehwm/2,
- zctx_set_sndhwm/2,
- zctx_set_rcvhwm/2,

See the manual for more informations.
Merge pull request #12 from benoitc/feature/czctx_fun

add options to set context options