Configuration name is wrong on README

[SantiagoValdez]

Hi!
Before hand, thank you for your extension, I really like it.

I found that there is a little mismatch between the configuration name given in the README, and the one that is actually read in config.ts file. In the README, the string is "pomodoro.tasks_file" but, in the config.ts you are accesing the property task_file. Like you can see in here:

 tasks_file: configuration.task_file || path.join(__dirname, `../../tasks.json`)

Thank you again for your effort. Hope this can help you.