Validate a name of theme

[tobynet]

Motivation

If the theme does not exist, the longcat causes panic.
This is not easy-to-understand for humankind.

• Add validation of theme name
• Add -themes option to list themes

ref. Supporting theme #12

Before

$ go run main.go -n 2 -H -t wlongcat
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x████████]

goroutine 1 [running]:
main.main()
        ████████████████████████████████████████████longcat/main.go:151 +0x4d5
exit status 2

After

$ go run main.go -n 2 -H -t wlongcat
████████ ████ theme does not exist: wlongcat
exit status 1

$ go run main.go -themes
longcat
tacgnol

[mattn]

Thanks