custom targets: Add a 'console' kwarg for long-running commands

Ninja buffers all commands and prints them only after they are
complete. Because of this, long-running commands such as `cargo
build` show no output at all and it's impossible to know if the
command is merely taking too long or is stuck somewhere.

To cater to such use-cases, Ninja has a 'pool' with depth 1 called
'console', and all processes in this pool have the following
properties:

1. stdout is connected to the program, so output can be seen in
   real-time
2. The output of all other commands is buffered and displayed after
   a command in this pool finishes running
3. Commands in this pool are executed serially (normal commands
   continue to run in the background)

This feature is available since Ninja v1.5

https://ninja-build.org/manual.html#_the_literal_console_literal_pool

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+## New kwarg `console` for `custom_target()`
+
+This keyword argument conflicts with `capture`, and is meant for commands
+that are resource-intensive and take a long time to finish. With the Ninja
+backend, setting this will add this target to [Ninja's `console`
+pool](https://ninja-build.org/manual.html#_the_literal_console_literal_pool),
+which has special properties such as not buffering stdout and serializing all
+targets in this pool.
+
+The primary use-case for this is to be able to run external package managers
+such as `cargo` to produce build artifacts. Without setting this, the user does
+not receive any feedback about what the package manager is doing, which can
+take a very long time to download and build packages.