Add write I/O to file

Add write I/O to file

This is a theorical feature listed in http://st.suckless.org/goals. All the
input/output of the terminal will be written to a file, which can be very
useful for debugging, and also allow interconnect st to other process
through named pipes.
---
 st.1 |    6 ++++++
 st.c |   14 +++++++++++++-
 2 files changed, 19 insertions(+), 1 deletion(-)