v3: Allow interpolation on "includes"

The idea is to allow manual inclusion of a OS-dependant Taskfile, since it's
not automatically included anymore.

testdata/includes/Taskfile.yml

@@ -13,6 +13,7 @@ includes:
   included_taskfile_with_dir:
     taskfile:  ./module2/Taskfile.yml
     dir:  ./module2
+  included_os: ./Taskfile_{{OS}}.yml
 
 tasks:
   default:
@@ -24,6 +25,7 @@ tasks:
       - task: included_taskfile_without_dir:gen_dir
       - task: included_with_dir:gen_file
       - task: included_taskfile_with_dir:gen_dir
+      - task: included_os:gen
 
   gen:
     cmds:

testdata/includes/Taskfile_darwin.yml

@@ -0,0 +1,4 @@
+version: '3'
+
+tasks:
+  gen: echo 'os' > os_include.txt

testdata/includes/Taskfile_linux.yml

@@ -0,0 +1,4 @@
+version: '3'
+
+tasks:
+  gen: echo 'os' > os_include.txt

testdata/includes/Taskfile_windows.yml

@@ -0,0 +1,4 @@
+version: '3'
+
+tasks:
+  gen: echo 'os' > os_include.txt