Add a command to encode a diagram

[ggrossetie]

$ kroki encode hello.dot
eNpLyUwvSizIUHBXqOZSUPBIzcnJ17ULzy_KSeGqBQCEzQka

$ cat hello.dot | kroki encode -
eNpLyUwvSizIUHBXqOZSUPBIzcnJ17ULzy_KSeGqBQCEzQka

$ echo "blockdiag {Kroki -> generates -> diagrams}" | kroki encode -
eNpLyslPzk7JTExXqPYuys_OVNC1U0hPzUstSixJLQZxQHJFibnFtQBEwA8V

Initial discussion: https://github.com/yuzutech/kroki/issues/3