prefix mods

[svanellewee]

Allow instances to specify the "BOT_CMD_PREFIX" before running.
This is to address
#42
#6
Example:

   export TELEGRAM_TOKEN=....
   export BOT_CMD_PREFIX='/'
   mkdir testtel ; cd testtel
   cat <<"EOF" > main.go
package main
import (
    "os"
    "github.com/go-chat-bot/bot/telegram"
    _ "github.com/go-chat-bot/plugins/catfacts"
    _ "github.com/go-chat-bot/plugins/catgif"
    _ "github.com/go-chat-bot/plugins/chucknorris"
    // Import all the commands you wish to use
)
func main() {
    telegram.Run(os.Getenv("TELEGRAM_TOKEN"), os.Getenv("DEBUG") != "")
}
EOF
go build -a
./testtel

[fabioxgn]

The ideal to solve this is to allow commands to be executed using the bot name on networks that support it, like @gobot help in slack. So I'm closing this PR as this doesn't solve this problem.