Improve nested variables support

Closes #76 #89 #77 #83

testdata/vars/Taskfile.yml

@@ -1,48 +0,0 @@
-default:
-  deps: [hello]
-
-hello:
-  cmds:
-    - echo {{.FOO}} > foo.txt
-    - echo {{.BAR}} > bar.txt
-    - echo {{.BAZ}} > baz.txt
-    - echo '{{.TMPL_FOO}}' > tmpl_foo.txt
-    - echo '{{.TMPL_BAR}}' > tmpl_bar.txt
-    - echo '{{.TMPL_FOO2}}' > tmpl_foo2.txt
-    - echo '{{.TMPL_BAR2}}' > tmpl_bar2.txt
-    - echo '{{.SHTMPL_FOO}}' > shtmpl_foo.txt
-    - echo '{{.SHTMPL_FOO2}}' > shtmpl_foo2.txt
-    - echo '{{.NESTEDTMPL_FOO}}' > nestedtmpl_foo.txt
-    - echo '{{.NESTEDTMPL_FOO2}}' > nestedtmpl_foo2.txt
-    - echo {{.FOO2}} > foo2.txt
-    - echo {{.BAR2}} > bar2.txt
-    - echo {{.BAZ2}} > baz2.txt
-    - echo '{{.TMPL2_FOO}}' > tmpl2_foo.txt
-    - echo '{{.TMPL2_BAR}}' > tmpl2_bar.txt
-    - echo '{{.TMPL2_FOO2}}' > tmpl2_foo2.txt
-    - echo '{{.TMPL2_BAR2}}' > tmpl2_bar2.txt
-    - echo '{{.SHTMPL2_FOO}}' > shtmpl2_foo.txt
-    - echo '{{.SHTMPL2_FOO2}}' > shtmpl2_foo2.txt
-    - echo '{{.NESTEDTMPL2_FOO2}}' > nestedtmpl2_foo2.txt
-    - echo {{.OVERRIDE}} > override.txt
-  vars:
-    FOO: foo
-    BAR: $echo bar
-    BAZ:
-      sh: echo baz
-    TMPL_FOO: "{{.FOO}}"
-    TMPL_BAR: "{{.BAR}}"
-    TMPL_FOO2: "{{.FOO2}}"
-    TMPL_BAR2: "{{.BAR2}}"
-    SHTMPL_FOO:
-      sh: "echo '{{.FOO}}'"
-    SHTMPL_FOO2:
-      sh: "echo '{{.FOO2}}'"
-    NESTEDTMPL_FOO: "{{.TMPL_FOO}}"
-    NESTEDTMPL_FOO2: "{{.TMPL2_FOO2}}"
-    OVERRIDE: "bar"
-
-invalid-var-tmpl:
-  vars:
-    CHARS: "abcd"
-    INVALID: "{{range .CHARS}}no end"