New version aims to simplify main vault and support deeper merging
strategies.  We'll see...

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.