Update slug format for post type

Posts no longer need to specify a full url for the resulting html.
Rather, the prefix portion of the url will be inferred from the
post's location in the source filesystem. The post slug now simply
represents a url friendly version of the title.

Additionally, updates all sample posts to use the new slug format.

content/posts/2016-01-09---Perfecting-the-Art-of-Perfection.md

@@ -3,7 +3,7 @@ title: Perfecting the Art of Perfection
 date: "2016-09-01T23:46:37.121Z"
 template: "post"
 draft: false
-slug: "/posts/perfecting-the-art-of-perfection/"
+slug: "perfecting-the-art-of-perfection"
 category: "Design Inspiration"
 tags:
   - "Handwriting"

content/posts/2016-01-12---The-Origins-of-Social-Stationery-Lettering.md

@@ -3,7 +3,7 @@ title: The Origins of Social Stationery Lettering
 date: "2016-12-01T22:40:32.169Z"
 template: "post"
 draft: false
-slug: "/posts/the-origins-of-social-stationery-lettering"
+slug: "the-origins-of-social-stationery-lettering"
 category: "Design Culture"
 description: "Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Vestibulum tortor quam, feugiat vitae, ultricies eget, tempor sit amet, ante."
 socialImage: "/media/image-3.jpg"

content/posts/2016-02-02---A-Brief-History-of-Typography.md

@@ -3,7 +3,7 @@ title: "A Brief History of Typography"
 date: "2016-02-02T22:40:32.169Z"
 template: "post"
 draft: false
-slug: "/posts/a-brief-history-of-typography/"
+slug: "a-brief-history-of-typography"
 category: "Design Inspiration"
 tags:
   - "Linotype"

content/posts/2017-18-08---The-Birth-of-Movable-Type.md

@@ -3,7 +3,7 @@ title: "Johannes Gutenberg: The Birth of Movable Type"
 date: "2017-08-18T22:12:03.284Z"
 template: "post"
 draft: false
-slug: "/posts/the-birth-of-movable-type/"
+slug: "the-birth-of-movable-type"
 category: "Typography"
 tags:
   - "Open source"

content/posts/2017-19-08---Humane-Typography-in-the-Digital-Age.md

@@ -3,7 +3,7 @@ title: Humane Typography in the Digital Age
 date: "2017-08-19T22:40:32.169Z"
 template: "post"
 draft: false
-slug: "/posts/humane-typography-in-the-digital-age/"
+slug: "humane-typography-in-the-digital-age"
 category: "Typography"
 tags:
   - "Design"

gatsby/on-create-node.js

@@ -8,10 +8,11 @@ const onCreateNode = ({ node, actions, getNode }) => {
 
   if (node.internal.type === 'MarkdownRemark') {
     if (typeof node.frontmatter.slug !== 'undefined') {
+      const dirname = getNode(node.parent).relativeDirectory;
       createNodeField({
         node,
         name: 'slug',
-        value: node.frontmatter.slug
+        value: `/${dirname}/${node.frontmatter.slug}`
       });
     } else {
       const value = createFilePath({ node, getNode });