feat: Remove source/posts/layouts flags

The main use cases for flags that mirror the config file
- One-off overriding.  The local setup might be different than the
  deployed.  This is more for cases like `dest`.  For the flags being
  removed, I cannot see why someone would need to override these config
  settings.
- No config file.  This just seems error prone.  I don't think this is a
  use case we are aiming to support.

So why remove?
- Less to test
- Less semantics to worry about defining.  Should these be relative to
  the config file, CWD, etc?

This is a step towards #284

BREAKING CHANGE: --source, --posts, --layouts flags have been removed.

src/bin/cobalt/main.rs

@@ -97,34 +97,13 @@ fn run() -> Result<()> {
                  .help("Config file to use [default: .cobalt.yml]")
                  .global(true)
                  .takes_value(true))
-        .arg(Arg::with_name("source")
-                 .short("s")
-                 .long("source")
-                 .value_name("DIR")
-                 .help("Source folder [default: ./]")
-                 .global(true)
-                 .takes_value(true))
         .arg(Arg::with_name("destination")
                  .short("d")
                  .long("destination")
                  .value_name("DIR")
                  .help("Destination folder [default: ./]")
                  .global(true)
                  .takes_value(true))
-        .arg(Arg::with_name("layouts")
-                 .short("l")
-                 .long("layouts")
-                 .value_name("DIR")
-                 .help("Layout templates folder [default: ./_layouts]")
-                 .global(true)
-                 .takes_value(true))
-        .arg(Arg::with_name("posts")
-                 .short("p")
-                 .long("posts")
-                 .value_name("DIR")
-                 .help("Posts folder [default: ./posts]")
-                 .global(true)
-                 .takes_value(true))
         .arg(Arg::with_name("drafts")
                  .long("drafts")
                  .help("Include drafts.")
@@ -293,30 +272,12 @@ fn run() -> Result<()> {
         Config::from_cwd(cwd)?
     };
 
-    config.source = matches
-        .value_of("source")
-        .or_else(|| global_matches.value_of("source"))
-        .map(str::to_string)
-        .unwrap_or(config.source);
-
     config.dest = matches
         .value_of("destination")
         .or_else(|| global_matches.value_of("destination"))
         .map(str::to_string)
         .unwrap_or(config.dest);
 
-    config.layouts = matches
-        .value_of("layouts")
-        .or_else(|| global_matches.value_of("layouts"))
-        .map(str::to_string)
-        .unwrap_or(config.layouts);
-
-    config.posts = matches
-        .value_of("posts")
-        .or_else(|| global_matches.value_of("posts"))
-        .map(str::to_string)
-        .unwrap_or(config.posts);
-
     config.include_drafts = matches.is_present("drafts");
 
     if global_matches.is_present("dump") {