fix(prismjs): new configuration parameter + update primjs

[dezounet]

Changes

• Disable/Enable primjs with .Site.Params.Prismjs.Enable
  • Hugo will not load prismjs files (css and js) if this is not enabled
• Update prismjs version to 1.24.1
• Remove trailing new line from primjs shortcode

This may address #113, and requires one to add a new section to its config.toml:

[params.prismjs]
  enable = true

Output sample

{{< prism line=true lang="go" >}}
package main
import "fmt"
func main() {
    fmt.Println("hello world")
}
{{< /prism >}}

Gives this:

[MKastuv]

I was about to open the issue about syntax highlighting, this could fix the problem.