fish: add page

Added the fish shell. Information included was based on other shells,
notably bash.

pages/common/fish.md

@@ -0,0 +1,27 @@
+# fish
+
+> A command-line shell for the 90's
+
+- Start interactive shell:
+
+`fish`
+
+- Execute a command:
+
+`fish -c "{{command}}"`
+
+- Run commands from a file:
+
+`fish {{file.fish}}`
+
+- Check a file for syntax:
+
+`fish --no-execute {{file.fish}}`
+
+- Display version information and exit:
+
+`fish --version`
+
+- Enable debug information (ranges from 0 to 128):
+
+`fish --debug-stack-frames={{NUMBER}}`