docs: add a new page with global flags and link to it from the command docs

In f544234 we removed the global flags from each command as it was
making each page very big and causing 1000s of lines of duplication in
the man page.

This change adds a new flags page with all the global flags on and
links each command page to it.

Fixes #3273

docs/content/commands/rclone.md

@@ -1,5 +1,5 @@
 ---
-date: 2019-06-15T12:00:42+01:00
+date: 2019-06-20T16:09:42+01:00
 title: "rclone"
 slug: rclone
 url: /commands/rclone/
@@ -29,6 +29,8 @@ rclone [flags]
   -h, --help   help for rclone
 ```
 
+See the [global flags page](/flags/) for global options not listed here.
+
 ### SEE ALSO
 
 * [rclone about](/commands/rclone_about/)	 - Get quota information from the remote.
@@ -79,4 +81,3 @@ rclone [flags]
 * [rclone tree](/commands/rclone_tree/)	 - List the contents of the remote in a tree like fashion.
 * [rclone version](/commands/rclone_version/)	 - Show the version number.
 
-###### Auto generated by spf13/cobra on 15-Jun-2019