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(-)

st.1

@@ -10,6 +10,8 @@ st \- simple terminal
 .RB [ \-w 
 .IR windowid ]
 .RB [ \-v ]
+.RB [ \-f
+.IR file ]
 .RB [ \-e
 .IR command ...]
 .SH DESCRIPTION
@@ -30,6 +32,10 @@ embeds st within the window identified by
 .B \-v
 prints version information to stderr, then exits.
 .TP
+.BI \-f " file"
+writes all the I/O to
+.I file
+.TP
 .BI \-e " program " [ " arguments " "... ]"
 st executes
 .I program