Panic when no or bad file supplied

[jake-scott]

The tool panics / segfaults when no "-f" option is given or if the file isn't parsable:

$ ./go-outline 
error: Could not parse file 
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x574c77]

goroutine 1 [running]:
main.main()
	/home/jacob/src/go/github.com/go-outline/main.go:63 +0x2c7


$ ./go-outline  -f foo
error: Could not parse file foo
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x574d39]

goroutine 1 [running]:
main.main()
	/home/jacob/src/go/github.com/go-outline/main.go:68 +0x2d9

I am about so submit a minor PR to fix.