Add update checker

[mildm8nnered]

Adds an update check facility to SwiftLint - addresses #5551

This has been added as a --check-for-updates command line option to the version, lint, and analyze commands.

It can also be enabled via the configuration file, as check_for_updates.

Although the usefulness of an opt-in option is limited, at least this provides a good story for how users can keep their installs up to date. And it could easily be changed to opt-out either here or at some later date.

The update checker code is pretty closely based on Periphery's implementation, and should probably be credited in the source file.

The update check is performed after all other processing, so linting will never be delayed, and the code is completely synchronous, as we do not have to worry about blocking the main thread.

The messaging for a new version is: A new version of SwiftLint is available: 0.51.1

We currently do not print any messaging if the update check fails, which is probably wrong.

% swiftlint help version
OVERVIEW: Display the current version of SwiftLint

USAGE: swiftlint version [--verbose] [--check-for-updates]

OPTIONS:
  --check-for-updates     Check whether a later version of SwiftLint is available.

%
% ./swiftlint version --check-for-updates
0.55.1
Your version of SwiftLint is up to date
%
%
% swiftlint help lint
OVERVIEW: Print lint warnings and errors

USAGE: swiftlint lint [<options>] [<paths> ...]

ARGUMENTS:
  <paths>                 List of paths to the files or directories to lint.

OPTIONS:
  --check-for-updates     Check whether a later version of SwiftLint is available.

[SwiftLintBot]

	1 Error
:no_entry_sign:	Could not build branch

	1 Warning
:warning:	If this is a user-facing change, please include a CHANGELOG entry to credit yourself! You can find it at CHANGELOG.md.

Here's an example of your CHANGELOG entry:

* Add update checker.  
  [mildm8nnered](https://github.com/mildm8nnered)
  [#issue_number](https://github.com/realm/SwiftLint/issues/issue_number)

note: There are two invisible spaces after the entry's text.

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