DC v1.0.0 - cross-platform D compiler install management tool

[Mihails]

Glad to announce first version of 
https://github.com/mihails-strasuns/dc suitable for public 
destruction. It is a small utility originally created to fulfill 
similar purpose to existing `install.sh` script but also with 
Windows support.

https://github.com/mihails-strasuns/dc/releases/tag/v1.0.0

It is provided as an independent statically linked binary which 
creates and maintain own toolchain directory with 
`bin`/`lib`/`imports` sub-directories and allows to quickly 
switch between compiler versions.

Two main use cases:

- Cross-platform CI, being able to just say `dc use ldc-1.14.0` 
in your test spec for both Linux and Windows
- Quicky and dirty switching between compiler versions on 
development machines.

Contrary to `install.sh` and DVM approach, DC changes active 
compiler globally instead of a current shell context only. This 
done intentionally to simplify integration with IDEs and other 
non-CLI tools - though temporary shell-only switching is also 
planned once I come with a decent design.

It is also possible (and encouraged) to put own development tools 
and static libraries in toolchain `bin` and `lib` folders 
accordingly to make them easily available without doing any 
system install.

Right now the tool is in "works for me" stage, any feedback is 
welcome.