Add include by anchor in preprocessor (partial include)

[CBenoit]

For issue #811.
I apologize for the late pull request.

I'm not super happy with
https://github.com/rust-lang-nursery/mdBook/pull/851/files#diff-d6ae283361f866777987f4630e148843R138
and
https://github.com/rust-lang-nursery/mdBook/pull/851/files#diff-182fe74cafd75eb44defddf4511f3591R70

Please, tell me what you think about the anchored syntax. I'll update the user guide after.

In current implementation, first anchor must match regex "ANCHOR:\s*[\w\d_-]+" and similarly the ending anchor must match "ANCHOR_END:\s*[\w\d_-]+". In included file:

/* ANCHOR: all */

// ANCHOR: component
struct Paddle {
    hello: f32,
}
// ANCHOR_END: component

////////// ANCHOR: system
impl System for MySystem { ... }
////////// ANCHOR_END: system

/* ANCHOR_END: all */

Then in the book:

```rust,no_run,noplaypen
{{#include file.rs:component}}
`` `

```rust,no_run,noplaypen
{{#include file.rs:system}}
`` `

```rust,no_run,noplaypen
{{#include file.rs:all}}
`` `

Lines containing anchor patterns inside the included anchor are ignored, as such the result is:

struct Paddle {
    hello: f32,
}

impl System for MySystem { ... }

struct Paddle {
    hello: f32,
}

impl System for MySystem { ... }

[nikomatsakis]

This would be so nice to have!

[CBenoit]

I would be happy to finish the work if I can get more feedback 👍

[ehuss]

I think this looks fine to me. Just needs a rebase and some docs.

@Dylan-DPC Do you have any thoughts on this?

[nikomatsakis]

I'm very excited, I'd love to see this land. FWIW, I reviewed the code briefly and thought it looked good.

[ehuss]

@CBenoit Do you think you'll be able to rebase this and add some docs? If not, that's ok, I can take it over if you don't have the time.

[CBenoit]

@ehuss I'm currently working on it! If everything rebase nicely it'll be finished soon.

[CBenoit]

Okay! Everything should be fine now 👍

[ehuss]

Great, thanks!