Allow to configure a baseline unit

[rxaviers]

Currently, it's possible to use this library and ignore large units by using the threshold configuration [1]. It should also be possible to ignore small units potentially by using a baseline (any better word?) configuration [2].

1:

const relativeTime = new RelativeTime();
relativeTime.format(aTwoYearsAgoDate));
// > '2 years ago'

RelativeTime.threshold.month = Infinity;
relativeTime.format(aTwoYearsAgoDate);
// > '24 months ago'

2:

relativeTime.format(new Date()));
// > "now"

RelativeTime.baseline.minute = 0
RelativeTime.threshold.second = -1
relativeTime.format(new Date()));
// > "this minute"

That could be accomplish with the following change:

diff --git a/src/relative-time.js b/src/relative-time.js
index b49e5eb..1fe5a1c 100644
--- a/src/relative-time.js
+++ b/src/relative-time.js
@@ -117,14 +117,15 @@ export default class RelativeTime {
 
 RelativeTime.bestFit = function(absDiff) {
   let threshold = this.threshold;
+  let baseline = this.baseline;
   switch(true) {
-    case absDiff.years > 0 && absDiff.months > threshold.month: return "year";
-    case absDiff.months > 0 && absDiff.days > threshold.day: return "month";
-    // case absDiff.months > 0 && absDiff.weeks > threshold.week: return "month";
-    // case absDiff.weeks > 0 && absDiff.days > threshold.day: return "week";
-    case absDiff.days > 0 && absDiff.hours > threshold.hour: return "day";
-    case absDiff.hours > 0 && absDiff.minutes > threshold.minute: return "hour";
-    case absDiff.minutes > 0 && absDiff.seconds > threshold.second: return "minute";
+    case absDiff.years >= baseline.year && absDiff.months > threshold.month: return "year";
+    case absDiff.months >= baseline.month && absDiff.days > threshold.day: return "month";
+    // case absDiff.months >= 1 && absDiff.weeks > threshold.week: return "month";
+    // case absDiff.weeks >= 1 && absDiff.days > threshold.day: return "week";
+    case absDiff.days >= baseline.day && absDiff.hours > threshold.hour: return "day";
+    case absDiff.hours >= baseline.hour && absDiff.minutes > threshold.minute: return "hour";
+    case absDiff.minutes >= baseline.minute && absDiff.seconds > threshold.second: return "minute";
     default: return "second";
   }
 };
@@ -138,6 +139,15 @@ RelativeTime.threshold = {
   second: 59 // at least 59 seconds before using minute.
 };
 
+RelativeTime.baseline = {
+  year: 1,
+  month: 1, // at least 2 months before using year. TODO
+  day: 1, // at least 6 days before using month. TODO
+  hour: 1, // at least 6 hours before using day. TODO
+  minute: 1 // at least 59 minutes before using hour. TODO
+};
+
 // TODO: Remove redundancy. The only reason this code is that ugly is to get
 // supported by globalize-compiler (which reads the static formatters).
 RelativeTime.initializeFormatters = function(globalize) {

Side-note: It's a good approach to enable "week", because it's possible to keep the existing behavior by configuring threshold and baseline.