New formula for Skyflash

**Package description**
- Name: skyflash
- Homepage: https://github.com/skycoin/skyflash
- Type: git
- Download URL: https://github.com/skycoin/skyflash/releases
- Version(s): 0.0.4 (to be released)
- Compiler info: <= @stdevPavelmc ?

**Rationale**
Skyflash project develops the command line and GUI tools for flashing Skybian binary images onto SD cards used in official Skyminers (i.e. Orange Pi Plus 2) and DIY nodes for [supported platforms](https://github.com/skycoin/skywire/blob/master/testnet_rules.md#hardware)

- ~Inherited dependency~
- ~Internal project decision~
- [x] This is a deliverable : `skycoin/skyflash`

**Project dependency matrix**
This package will be used in the following projects

- [x] `skycoin/skywire`
- [x] `skycoin/skywire-discovery`
- [x] `skycoin/skywire-autoupdater`
- [x] `skycoin/skybian`
- [x] `skycoin/skyflash`

**Usage patterns**
- `skycoin/skywire` : image binaries downloaded from here (but already bundled in Skywire images)
- `skycoin/skywire-discovery` : included in Skybian images ( @stdevPavelmc cmiiw )
- `skycoin/skywire-autoupdater` : included in Skybian images ( @stdevPavelmc cmiiw )
- `skycoin/skybian` : images released by this project are used for flashing Skyniner SD cards
- `skycoin/skyflash` : download release artifacts from here

**Additional context**
@stdevPavelmc , anything else ?