Add {{#shiftinclude}} to L/R shift on include

Syntax is the same as {{#include}} except with a shift value and colon
before the remaining arguments, e.g.

  {{#include -2:somefile.rs:myanchor}}

A positive value for the shift prepends spaces to each line.

A negative value for the shift removes chars from the beginning of each
line (including non-whitespace chars, although this will emit an error
log).

Possibly helpful/relevant for:
- rust-lang#1564: #include with indented
- rust-lang#1601: option to remove indentation of included file snippets

Author: daviddrysdale

guide/src/format/mdbook.md

@@ -211,6 +211,18 @@ This is the full file.
 
 Lines containing anchor patterns inside the included anchor are ignored.
 
+## Including a file but changing its indentation
+
+Sometimes it is useful to include a file or part of a file but with the indentation shifted,
+using the following syntax:
+
+```hbs
+\{{#shiftinclude -2:file.rs}}
+```
+
+A positive number for the shift will prepend spaces to all lines; a negative number will remove
+the corresponding number of characters from the beginning of each line.
+
 ## Including a file but initially hiding all except specified lines
 
 The `rustdoc_include` helper is for including code from external Rust files that contain complete