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 to:
- rust-lang#1564: #include with indented
- rust-lang#1601: option to remove indentation of included file snippets