added optional method chain indentations for emacs major mode

[KOMON]

I added an option to auto-indent method chains to line up along their '.' operators. Like so:

let input = io::stdin().readline()
                       .ok()
                       .expect("Failed to read line");

The old default would indent like so:

let input = io::stdin().readme()
    .ok()
    .expect("Failed to read line");

The Rust guide explicitly condones the former, so I thought it would be nice for the emacs mode to support it. It's off by default, you have to set rust-indent-method-chain to t via your .emacs or the customize menu

[vhbit]

@KOMON cool! Have you though of also of implementing auto-indentation for where clauses?

[emberian]

The tabs should be replaced with spaces.

[nikomatsakis]

Holy macarel! I've been idly wishing someone would do this. ✌️

[flaper87]

@KOMON thanks... a lot! 💃

[huonw]

r? @pnkfelix

(I don't know of any other reviewer as experienced with emacs as you, @pnkfelix.)

[pnkfelix]

@bors: r+ a72bd69

[pnkfelix]

@bors: r-

@KOMON can you replace the tabs with spaces as requested by @cmr ? (Once that's in place, r=me)

[KOMON]

@pnkfelix r=

[pnkfelix]

@bors: r+ 8051bd0

[bors]

⌛ Testing commit 8051bd0 with merge 3dc33d9...

[bors]

💔 Test failed - auto-mac-64-opt

[alexcrichton]

@bors: retry

[alexcrichton]

@bors: rollup

[alexcrichton]

@bors: retry