diff: allow base switching between local or remote

[odeke-em]

Fixes #506

Allows users to switch decide if local
is the base otherwise use remote.

remote as the base, local as the other

$ drive diff --base-local=false ls.txt 
File: /diff-test/ls.txt
* local:          2015-11-26T17:32:49.000Z                
* remote:         2015-11-26T17:32:09.000Z                
ls.txt
****

--- .xtmp5577006791947779410.tmp537523880   2015-11-29 20:11:18.000000000 -0700
+++ /Users/emmanuelodeke/emm.odeke@gmail.com/diff-test/ls.txt   2015-11-26 10:32:49.000000000 -0700
@@ -1,15 +1,6 @@
-02 Downtown (feat. Kidd Kidd).m4a
-About_A_Girl.mp3
-drop.avi
-sample.3gp
-sample.m4a
-sample.midi
-sample.wav
-sample_3GPP2.3g2
-sample_iPod.m4v
-sample_iTunes.mov
-sample_mpeg2.m2v
-sample_mpeg4.mp4
-violin07.aif
-violin12.aif
-zips
+LICENSE
+README.md
+cache.go
+cache_test.go
+exports.go
+swift

local as the base, remote as the other

$ drive diff --base-local ls.txt 
File: /diff-test/ls.txt
* local:          2015-11-26T17:32:49.000Z                
* remote:         2015-11-26T17:32:09.000Z                
ls.txt
****

--- /Users/emmanuelodeke/emm.odeke@gmail.com/diff-test/ls.txt   2015-11-26 10:32:49.000000000 -0700
+++ .xtmp5577006791947779410.tmp658721850   2015-11-29 20:11:22.000000000 -0700
@@ -1,6 +1,15 @@
-LICENSE
-README.md
-cache.go
-cache_test.go
-exports.go
-swift
+02 Downtown (feat. Kidd Kidd).m4a
+About_A_Girl.mp3
+drop.avi
+sample.3gp
+sample.m4a
+sample.midi
+sample.wav
+sample_3GPP2.3g2
+sample_iPod.m4v
+sample_iTunes.mov
+sample_mpeg2.m2v
+sample_mpeg4.mp4
+violin07.aif
+violin12.aif
+zips