User data as store flow path

[joaohf]

compute store flow path based on user_data folder

There are some enviroments whe erlang-red is running as a non-privileged
user. Thus, if erlang-red is installed on /usr/ path it will not have
the correct permissions when writing flows to disk.

This change allows erlang-red compute a store flow path based on:

1. if flow_store application property has been configured, erlang-red
   will use that value. Otherwise:

2. get `user_data` [1] from filename:basedir/2 as a base folder for
   storing flows. e.g.: in my host (linux based) the store folder is
   /home/joaohf/.local/share/erlang-red/

1: https://www.erlang.org/doc/apps/stdlib/filename.html#basedir/3

[joaohf]

Hi @gorenje let me know what you think. I tried to describe my use case and a patch to accommodate that situation.

[gorenje]

Hi @joaohf

Looks good to me - I'll test locally and then merge here. Not sure about why the tests are failing hence I want to have a look on my local machine first.

Cheers!