Streamline version updates

Currently, releasing a new version of e.g. dapp requires:

* updating the [nix package](https://github.com/dapphub/dapptools/blob/master/src/dapp/default.nix#L7)
* updating [`dapp --version`](https://github.com/dapphub/dapptools/blob/master/src/dapp/libexec/dapp/dapp---version#L2)
* updating the [`CHANGELOG`](https://github.com/dapphub/dapptools/blob/master/src/dapp/CHANGELOG.md)
* pushing a git tag

This is error-prone and has already resulted in a few [mistakes](https://github.com/dapphub/dapptools/blob/5ee9a77f14d8e50a0bc420b402047f354171ece6/src/dapp/CHANGELOG.md#0151---2019-03-05).

One way to walk towards a more manageable solution is to institute a `VERSION` file for each tool, read by both the nix package and the `--version` subcommand. This would allow us to eliminate the first two steps above.

A next step could be to create a script that bumps the version in `VERSION` and performs operations/sanity checks:

* adds a section to the changelog
* creates a branch named `dapp/x.y.z`
* ...

/cc @nanexcool @desaperados