Use ANSI color code to represent added and removed line.

[bjacotg]

By default, the difference summary will color the extra lines. Extra lines in actual will be red while extra line in expected will be blue.

For instance the error from verify_that(vec![1,2,3,4], eq(vec![1,2,5,4])) will look like:

Error: Value of: vec![1, 2, 3, 4]
Expected: is equal to [1, 2, 5, 4]
Actual: [1, 2, 3, 4],
  which isn't equal to [1, 2, 5, 4]
Difference:
 [
     1,
     2,
+    3, // <---- this is in red
-    5, // <---- this is in blue
     4,
 ]

The original behavior (= no color) can still be triggered by setting the environment variable GTEST_RUST_NO_COLOR to any value.

This PR also refactor the summarize logic in its own file summarize_diff.rs

Note: You can comment on github/markup#1440 if you would like to this PR to have nice color instead of comments.

Partial work on #221