…riendly

The shell scripts no longer depend on knowing the "flag string" to
signal a change of directory. Instead, they now all rely on the
output_parse script to output the directory, if it exists, or the empty
string otherwise, which is more predictable (and would have avoided
0b51e8f from the start).

When a user writes `quik <tab>`, PowerShell autocompleted this to `quik
quik`, because it made no distinction between this sequence of inputs
and `quik <tab>`. The expansion to `quik quik` was wrong, because doing
`quik <tab>` was being passed to the autocomplete python script as
`quik<tab>` (returning "quik", to mean leave the word as is). To fix
this, I just check if the cursor's position is greater than the
autocomplete string (which means there are spaces between the cursor and
the string to autocomplete, which is trimmed), and if so add a trailing
space, hinting the autocomplete python script that following words
should be suggested.

The output_parse script uses a graph to decide autocompletion.
Previously, since the autocomplete request could pass in either, e.g.,

    quik --

or just

    --

I took the simple but brutish approach of defining two root nodes
("root" and "quik"), and just connecting the first level of the tree to
each of the nodes.

However, this was now breaking for PowerShell, where calling quik with
an alias, like q, passed into the completion script, e.g.,

    q

for which the defined aliases should be suggested, but no such root node
exists. So, I've introduced two changes:

 * Instead of multiple root nodes, there is now a single root node
   ("root"), and the concept of tree aliases, i.e., node names that
   resolve to a particular node.
   Aliases take precedence over node names, so nodes and aliases need to
   have different names.
   "quik" becomes an alias to "root".
 * An optional `--alias` argument gives the name with which the command
   is being called; this allows us to simply alias this name to "root"
   as well, to get correct autocompletion.

The next commit will integrate this behaviour into PowerShell. (Other
shells don't need this, as bash takes care of the aliased name, and
command prompt doesn't natively support autocompetion.)

A new function, Register-QuikAlias, has been defined, which
automatically sets up both the alias (via Set-Alias) and autocomplete
for that alias (via Register-ArgumentCompeter).

This lets the user register new aliases (like q) with

    Register-QuikAlias -Name q

Previously, a more backward compatible mechanism was used, since
$PSScriptRoot was only introduced in PowerShell 3. However, the previous
solution didn't always work. (In my particular case, for example,
using terminalizer led to the shell not being able to find the script's
source location, and assuming it to be the current working directory.)

Since backward compatibility with PowerShell <= 3 is not that important
(Windows currently ships with PS4 and PS7 is the latest version at the
time of writing), I just replaced the backward compatible code with
$PSScriptRoot, which fixed the issue.