establish a tw-install.rc file

This file contains all essential user-variables in one place.
Customizable with sane defaults, and help in the comments.
some examples might be;
```
### tw-install.rc configuration file
TASKDIR=~/.task
TASKDDIR=/var/task
PLATFORM=linux
MOBILE_PLATFORM=android
MOBILE_SOURCE=fdroid
PACKAGE_MANAGER=apt
HOOK_DIR=~/.task/hooks
TMP_DIR=~/tmp
TASKWIKI_DIR=~/.task/wiki
TASKNOTES_DIR=~/.task/notes
TASKTHEME_DIR=~/.task/theme
BIN_DIR=~/.local/bin
WEB_DIR=~/var/www/something
BACKUP_DIR=~/backup/taskwarrior
BACKUP_FMT=zip
EDITOR=vim
```
that sort of thing,