Add commands to layout

Add the ability to run commands on loading a layout:
```
  - direction: Horizontal
    split_size:
      Percent: 50
    run:
      command: {cmd: htop}
```
or respectively:
```
  - direction: Horizontal
    split_size:
      Percent: 50
    run:
      command: {cmd: htop, args: ["-C"]}
```

In order to specify the difference of commands and
plugins now the plugins need to be under the `run` section:
```
  - direction: Vertical
    split_size:
      Fixed: 2
    run:
      plugin: status-bar
```
This also means that this is a breaking change for people
that already have a custom layout.

Example layouts:
```
---
direction: Vertical
parts:
  - direction: Horizontal
    split_size:
      Percent: 50
    parts:
      - direction: Vertical
        split_size:
          Percent: 50
      - direction: Vertical
        split_size:
          Percent: 50
        run:
          command: {cmd: htop}
  - direction: Horizontal
    split_size:
      Percent: 50
    run:
      command: {cmd: htop}
```
and:
```
---
direction: Horizontal
parts:
  - direction: Vertical
    split_size:
      Fixed: 1
    run:
      plugin: tab-bar
  - direction: Vertical
    parts:
    - direction: Vertical
      parts:
      - direction: Vertical
        split_size:
          Percent: 50
        run:
          command: {cmd: htop}
      - direction: Vertical
        split_size:
          Percent: 50
        run:
          command: {cmd: htop, args: ["-C"]}
  - direction: Vertical
    split_size:
      Fixed: 5
  - direction: Vertical
    split_size:
      Fixed: 2
    run:
      plugin: status-bar
```

closes #551, closes #284

2 files changed, 50 insertions, 0 deletions

diff --git a/example/run_htop_layout.yaml b/example/run_htop_layout.yaml
new file mode 100644
index 000000000..949435d69
--- /dev/null
+++ b/example/run_htop_layout.yaml
@@ -0,0 +1,20 @@
+---
+direction: Vertical
+parts:
+  - direction: Horizontal
+    split_size:
+      Percent: 50
+    parts:
+      - direction: Vertical
+        split_size:
+          Percent: 50
+      - direction: Vertical
+        split_size:
+          Percent: 50
+        run:
+          command: {cmd: htop}
+  - direction: Horizontal
+    split_size:
+      Percent: 50
+    run:
+      command: {cmd: htop}
diff --git a/example/run_htop_layout_with_plugins.yaml b/example/run_htop_layout_with_plugins.yaml
new file mode 100644
index 000000000..99f72edbe
--- /dev/null
+++ b/example/run_htop_layout_with_plugins.yaml
@@ -0,0 +1,30 @@
+---
+direction: Horizontal
+parts:
+  - direction: Vertical
+    split_size:
+      Fixed: 1
+    run:
+      plugin: tab-bar
+  - direction: Vertical
+    parts:
+    - direction: Vertical
+      parts:
+      - direction: Vertical
+        split_size:
+          Percent: 50
+        run:
+          command: {cmd: htop}
+      - direction: Vertical
+        split_size:
+          Percent: 50
+        run:
+          command: {cmd: htop, args: ["-C"]}
+  - direction: Vertical
+    split_size:
+      Fixed: 5
+  - direction: Vertical
+    split_size:
+      Fixed: 2
+    run:
+      plugin: status-bar