This PR adds the remaining pieces needed for the `bevy build` command.

The command is a wrapper around `cargo build`, but with an additional
`web` sub command which makes it a lot easier to build an app for the
browser.
I also plan to add additional features in the feature, like #68.

The PR contains the following changes:

- Added `cargo install` wrapper to install a program if it's missing
(prompting the user).
- Added helper to obtain the current package name via `Cargo.toml`.
There are likely better ways to do this (perhaps via `cargo metadata`),
but I already had this implemented and we can improve it in a follow-up
if needed.
- Added helper to determine the active compilation profile, which is
needed to determine the path to the build artifact.
- Implemented the build command itself.