Each mask is appended to DOTROOT to create an alternative DOTROOT path,
and finally to link target.  Therefore, you can have shared structure
like this:

    dotroot/fooapp
    dotroot/barapp
    dotroot.config/janeapp
    dotroot.config/aliceapp
    dotroot.local/share/janeapp
    dotroot.local/share/fooapp

The first two examples work just like before, but if masks ".config" and
".local/share" are defined, the rest is treated specially.  Final
mapping looks like this:

    ~/.fooapp -> dotroot/fooapp
    ~/.barapp -> dotroot/barapp
    ~/.config/aliceapp -> dotroot.config/aliceapp
    ~/.config/janeapp -> dotroot.config/janeapp
    ~/.local/share/fooapp-> dotroot.local/share/fooapp
    ~/.local/share/janeapp -> dotroot.local/share/janeapp

This allows for quite a good flexibility of how you define your shared
structure to reflect habits of many common applications while retaining
ease of deplyment.

Note that:

*   `.config` and `.local/share` masks are defined by default

*   you can provide your own by using one or more `--mask` arguments;
    this, however, removes the default ones

**Warning:** Potential clases between masks like ".local/share and
".local" are **not** solved within script.  Avoid that.  Currently
it's an undefined behavior.