Display where is current cache folder

[non-descriptive]

When running gitui with -l/--logging parameter it is unclear where is the log stored. So it would be good to somehow display what is current cache directory is demangled from all the environment variables. There are couple ways to display it:

1. When we call gitui --help display it near --logging description, something like

# on Linux
  -l, --logging                Stores logging output into a cache directory (/home/user/.cache/gitui)
# on Windows
  -l, --logging                Stores logging output into a cache directory (C:\Users\user\.cache\gitui)

2. We can add extra command to display whatever paths gitui uses.

$ gitui --paths
home_dir=/home/user/
cache_dir=/home/user/.cache/gitui
...

or maybe

$ gitui --env
GIT_DIR=<not set>
GIT_WORKTREE=<not set>
home_dir=/home/user/
cache_dir=/home/user/.cache/gitui

3. Finally when running with --logging we can just yield sign

Log will be stored in: /home/user/.cache/gitui/gitui.log

and we will know log path even if gitui panics somehow.

[extrawurst]

good suggestion. I went for option 3 as a low hanging fruit