added optional path in --update

[cosmos1721]

in continuation of PR #535
as per your suggestions, an optional add to path has been added.
i.e. auto-cpufreq --update=path/to/dir

i was expecting to use:
auto-cpufreq --update path/to/dir
but there are some unknown issues

[AdnanHodzic]

1. Let's set to use /opt/auto-cpufreq/source as the default path as part of --update as part of this PR, and then having ability to set "custom path" can be made as part of one of following iterations.

Besides this I'll leave couple of comments for the code itself, as I found some bugs.

2. What's the proper mechanism for testing --update changes? As changing i.e VERSION = "1.9.8" line in setup.py had no impact, and even changing auto-cpufreq-installer file to trick it into that there are updates:

    # Fetch the latest release information from GitHub API
    #latest_release=$(curl -s "https://api.github.com/repos/$repository/releases/latest")
    # Extract the latest release version without using jq
    #latest_version=$(echo "$latest_release" | grep -o '"tag_name": "[^"]*' | cut -d'"' -f4)
    latest_version=v1.9.9 # same case with v2.0
    # Get the current version of auto-cpufreq
      #installed_version=$(pip list | awk '/auto-cpufreq/ {print $2}')
    installed_version=$(grep -oP "(?<=__requires__ = 'auto-cpufreq==)\d+(\.\d+)+" /opt/auto-cpufreq/venv/bin/auto-cpufreq)

kept returning:

sudo auto-cpufreq --update 
auto-cpufreq is up to date

[AdnanHodzic]

I don't think we need line 262 & 263, as this directory would've been created above if it doesn't exist.

[AdnanHodzic]

LGTM, thank you for your contribution!