Added log feature

Added "--log <filename>" option which writes all activity to specified
file.

src/log.c

@@ -0,0 +1,62 @@
+/*
+ * Go TTY - The Really Simple Terminal Application
+ *
+ * Copyright (c) 2014-2016  Martin Lund
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <errno.h>
+#include "gotty/options.h"
+#include "gotty/print.h"
+
+static FILE *fp;
+static bool error = false;
+
+void log_open(char *filename)
+{
+    fp = fopen(filename, "w+");
+
+    if (fp == NULL)
+    {
+        error = true;
+        exit(EXIT_FAILURE);
+    }
+}
+
+void log_write(char c)
+{
+    if (fp != NULL)
+        fputc(c, fp);
+}
+
+void log_close(void)
+{
+    if (fp != NULL)
+        fclose(fp);
+}
+
+void log_exit(void)
+{
+    if (option.log)
+        log_close();
+
+    if (error)
+        printf("Error: Could not open log file %s (%s)\n", option.log_filename, strerror(errno));
+}