Use UserCacheDir and UserConfigDir instead of custom implementation

Go have `os.UserCacheDir()` which added in 1.11

> UserCacheDir returns the default root directory to use for user-specific cached data. Users should create their own application-specific subdirectory within this one and use that.

> On Unix systems, it returns $XDG_CACHE_HOME as specified by https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html if non-empty, else $HOME/.cache. On Darwin, it returns $HOME/Library/Caches. On Windows, it returns %LocalAppData%. On Plan 9, it returns $home/lib/cache.

> If the location cannot be determined (for example, $HOME is not defined) or the path in $XDG_CACHE_HOME is relative, then it will return an error.

Go also have `os.UserConfigDir()` which added in 1.13

> UserConfigDir returns the default root directory to use for user-specific configuration data. Users should create their own application-specific subdirectory within this one and use that.

> On Unix systems, it returns $XDG_CONFIG_HOME as specified by https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html if non-empty, else $HOME/.config. On Darwin, it returns $HOME/Library/Application Support. On Windows, it returns %AppData%. On Plan 9, it returns $home/lib.

> If the location cannot be determined (for example, $HOME is not defined) or the path in $XDG_CONFIG_HOME is relative, then it will return an error.

It will be good to utilize this for `config.CacheDir()` and `config.ConfigDir()` instead of custom implementations.