Fix custom templates for messages

[strangeman]

• Fix https://github.com/inCaller/prometheus_bot/issues/13 (Flood by "Error sending message, checkout logs" messages)
• Fix https://github.com/inCaller/prometheus_bot/issues/14 (Template works only from current directory)
• Minor refactoring of template logic to avoid code repetition

FYI: I used html/template instead of txt/template for proper escaping, but it not fully solve the problem. To avoid a 503 Bad Request from Telegram server you should follow this requirements to the HTML code: https://core.telegram.org/bots/api#html-style

[hryamzik]

Great, thank you!