Closes #2186.

As requested in https://github.com/Homebrew/homebrew-core/issues/9316.

These can now be done with just `brew upgrade`, like other upgrades.

Fixes #1818.

Fixes #1817.

Unfortunately `brew linkapps` cannot behave nicely with e.g. Spotlight
using either aliases or symlinks and Homebrew formulae do not build
"proper" `.app` bundles that can be relocated. Instead, please consider
using `brew cask` and migrate formulae using `.app`s to casks.

GitHub now nicely generates a documentation site for us at
http://brew.sh/brew based on our docs folder. Optimise the output of
this and the GitHub docs directory for readability and the various user
groupings.

When reproducing issues with software that hasn’t been bottled yet on
your version of macOS it can sometimes be helpful to use `or_later`
bottle functionality i.e. just use the bottle for the latest version of
macOS available. This maps well to the existing `--force-bottle`
argument so it will now act as if the latest bottle has a `or_later`
ending.

meson is quickly gaining popularity as build system, in combination with
ninja. Several Gnome projects for example are currently transitioning
from autotools to meson, mostly because it allows for Visual Studio
builds, which is impossible to accomplish with autotools.

In order to facilitate generating meson based Formulas, I added support
for meson to brew-create.

For user-supplied PR message.

...and update man pages where applicable

Outputs the merged pull requests on Homebrew/brew between two Git refs.

Write the README but also a `.travis.yml` file (and in future perhaps a
`Jenkinsfile`).

These are the ones that are auto-installed without tapping and it makes
sense to make them more prominent in our documentation as we've
offloaded various functionality over the years to each of them.

This option is used to test updating between tags.

As requested in #1199 this updates the documentation accordingly for
that change.

Add some missed changes. I'll add a CI check for this at a future point.