feat(front): Set published_date from filename

This is inspired by #197 and Jekyll.

Jekyll gets the filename from a file.  The main difference between
Jekyll and this change is that we aren't supporting 2-digit years.

In #197, the idea was brought up of extracting the date from the
directory structure.  I like this approach.  I could imagine having at
least year directories (don't blog enough to make day directories worth
while).  The challenge comes in how to detect this.

src/frontmatter.rs

@@ -1,7 +1,9 @@
 use std::path;
 use std::collections::HashMap;
 
+use chrono::Datelike;
 use liquid;
+use regex;
 
 use error::Result;
 use datetime;
@@ -286,6 +288,11 @@ impl FrontmatterBuilder {
             self.title = Some(title);
         }
 
+        if self.published_date.is_none() {
+            let file_stem = file_stem(relpath);
+            self.published_date = extract_date(file_stem);
+        }
+
         self
     }
 
@@ -377,6 +384,33 @@ fn file_stem_path(p: &path::Path) -> String {
         .unwrap_or_else(|| "".to_owned())
 }
 
+fn extract_date(stem: String) -> Option<datetime::DateTime> {
+    lazy_static!{
+       static ref DATE_PREFIX_REF: regex::Regex = regex::Regex::new(r"^(\d{4})-(\d{1,2})-(\d{1,2})[- ].*$").unwrap();
+    }
+
+    DATE_PREFIX_REF.captures(&stem).and_then(|caps| {
+        let year: i32 = caps.get(1)
+            .expect("unconditional capture")
+            .as_str()
+            .parse()
+            .expect("regex gets back an integer");
+        let month: u32 = caps.get(2)
+            .expect("unconditional capture")
+            .as_str()
+            .parse()
+            .expect("regex gets back an integer");
+        let day: u32 = caps.get(3)
+            .expect("unconditional capture")
+            .as_str()
+            .parse()
+            .expect("regex gets back an integer");
+        datetime::DateTime::default()
+            .with_year(year)
+            .and_then(|d| d.with_month(month))
+            .and_then(|d| d.with_day(day))
+    })
+}
 
 #[cfg(test)]
 mod test {
@@ -389,6 +423,74 @@ mod test {
         assert_eq!(actual, "___filE-worlD-__09___");
     }
 
+    #[test]
+    fn extract_date_empty() {
+        assert_eq!(extract_date("".to_owned()), None);
+    }
+
+    #[test]
+    fn extract_date_none() {
+        assert_eq!(extract_date("First Blog Post".to_owned()), None);
+    }
+
+    #[test]
+    fn extract_date_out_of_range_month() {
+        assert_eq!(extract_date("2017-30-5 First Blog Post".to_owned()), None);
+    }
+
+    #[test]
+    fn extract_date_out_of_range_day() {
+        assert_eq!(extract_date("2017-3-50 First Blog Post".to_owned()), None);
+    }
+
+    #[test]
+    fn extract_date_single_digit() {
+        assert_eq!(extract_date("2017-3-5 First Blog Post".to_owned()),
+                   Some(datetime::DateTime::default()
+                            .with_year(2017)
+                            .unwrap()
+                            .with_month(3)
+                            .unwrap()
+                            .with_day(5)
+                            .unwrap()));
+    }
+
+    #[test]
+    fn extract_date_double_digit() {
+        assert_eq!(extract_date("2017-12-25 First Blog Post".to_owned()),
+                   Some(datetime::DateTime::default()
+                            .with_year(2017)
+                            .unwrap()
+                            .with_month(12)
+                            .unwrap()
+                            .with_day(25)
+                            .unwrap()));
+    }
+
+    #[test]
+    fn extract_date_double_digit_leading_zero() {
+        assert_eq!(extract_date("2017-03-05 First Blog Post".to_owned()),
+                   Some(datetime::DateTime::default()
+                            .with_year(2017)
+                            .unwrap()
+                            .with_month(3)
+                            .unwrap()
+                            .with_day(5)
+                            .unwrap()));
+    }
+
+    #[test]
+    fn extract_date_dashed() {
+        assert_eq!(extract_date("2017-3-5-First-Blog-Post".to_owned()),
+                   Some(datetime::DateTime::default()
+                            .with_year(2017)
+                            .unwrap()
+                            .with_month(3)
+                            .unwrap()
+                            .with_day(5)
+                            .unwrap()));
+    }
+
     #[test]
     fn frontmatter_title_from_path() {
         let front = FrontmatterBuilder::new()