Fix configuration use in start command

[shantanuraj]

What does this PR do?

• Start command should work even if config doesn't exist

  let config_path = config::locate::locate_config_path()?;
  let track_config = config::parser::get_config_from_file(config_path)?;
  let default_time_entry = track_config.get_default_entry(entities.clone())?;

  These established a hard dependency on the config infrastructure.
  We fix this, by only following the config code branch if a configuration
  path can be located.

  Caveat: This silences the error if a configuration is found but cannot
  be parsed. I'm treading on the side of simplicity over correctness here.
  In future we might treat the configuration error as a critical error.

• Simplify default config file
  Previously our default config for toggl config --init enforced
  the use of git.
  Such that, even for non-vcs tracked paths we would use a configuration
  that tries to resolve it's values from git which errors out.

  I think in general the wildcard match rule [*] should be agnostic of vcs.

  This changes the default to use {{current_dir}} which is guarenteed
  to work at any path the toggl command is invoked as is a much better
  default.

Note: This is my configuration block for the wildcard match.

['*']
tags = ["{{current_dir}}"]
billable = true

Context

Closes #84