delete socket file after accidental crash

[strogiyotec]

If pomo was closed because of system crash or termination then .sock file still exists which doesn't allow a new pomo instance to start with the following error

bind: address already in use

How to reproduce

1. Start pomo
2. Close terminal

I added new conditions to check if .sock file already exists and if another pomo instance is listening the .sock file

[kevinschoon]

This PR looks great but I haven't been able to verify the ability to delete multiple tasks yet. Maybe we should add a test case for it?

[strogiyotec]

@kevinschoon I will create a test case then, thank you for response

[strogiyotec]

@kevinschoon I wasn't able to run tests for existing app
How to reproduce

go test
# github.com/kevinschoon/pomo [github.com/kevinschoon/pomo.test]
./runner_test.go:21:30: too many arguments in call to NewTaskRunner
	have (*Task, *Store, NoopNotifier)
	want (*Task, *Config)
FAIL	github.com/kevinschoon/pomo [build failed]

Will try to fix this test as well

[strogiyotec]

@kevinschoon I think it does make sense to create different pull requests for different features, let me create a pull request to fix sock file problem , after that I will create another one where I will fix runner_test.go file and third one where I will add multiple deletion feature