Add a -version flag

[mytlogos]

Steps for Reproduction

1. Run pihole_exporter -version or any other possible equivalents

Expected behavior:
Return the current pihole_exporter version, at least just a line like pihole_exporter version x.x.x, at best something like what node_exporter --version outputs:

node_exporter, version 1.4.0 (branch: HEAD, revision: 7da1321761b3b8dfc9e496e1a60e6a476fec6018)
  build user:       root@83d90983e89c
  build date:       20220926-12:33:37
  go version:       go1.19.1
  platform:         linux/arm64

Actual behavior:
Outputs the cli help as an unknown flag was given.

Platforms:

• every supported (at least arm64)

Include browser, operating system and respective versions

Versions:

Which versions are you running?
I do not know, but most likely the current latest, version 0.3.0.

Additional Info

Currently one cannot know what version of this software is being used, as it does not has a "-version" or "--version" flag, nor a /version endpoint nor does it expose its version in the /metrics endpoint.
Knowing the current used version is very helpful in e.g. deciding if a upgrade is necessary (e.g. for ansible).
As far as i know, node_exporter uses the prometheus/commons library to provide the output of the --version flag and it somehow works maybe with make? and the VERSION file in its repository, though i personally could not make out its specifics.

[mytlogos]

You can embed the current version when building, by using the ldflag "-X":

// main.go
var (
    version = ""
    commit = ""
    date = ""
)

go build -ldflags '-X main.version=0.5.0 -X main.commit=hash -X main.date=2023 -X main.builtBy=eko'

The -X ldflag sets variable values (not const i think), to specific values.
The variables is loaded by an import path like "main" for package main in main.go or for example github.com/eko/pihole-exporter/config.

I could try to provide a PR, by integrating it also in github action and the dockerfile.