Updated styles and --keep option

Changes in v0.1.1:

• the target file defaults to current directory now. This resolves the issue of images/assets not appearing when using relative paths
• to keep directories clean, the target file .mark.html is removed automatically after opening in the browser
• use the --keep option if you want to keep the target file around
• syntax highlighting has been added to the default template
• syntax highlighting is only added to fenced code blocks an explicitly set language
• updated the CSS to be a bit more modern

To install:

• install Crystal
• check out repo and run make build
• binary will be at bin/mark