Change: keep actual text of keyword from scenario

The typesetting should preserve the actual keyword or alias in the
source. Previously, if source had this:

```
given foo
and bar
```

it got typeset as if were:

```
given foo
given bar
```

Also, change subplot.md to use alias when possible.

1 files changed, 7 insertions, 3 deletions

diff --git a/src/error.rs b/src/error.rs
index 3d28d3b..0a79cf6 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -89,15 +89,19 @@ pub enum SubplotError {
     #[error("first scenario is before first heading")]
     ScenarioBeforeHeading,
 
-    /// Unknown scenario step keyword
+    /// Step does not have a keyword.
+    #[error("step has no keyword: {0}")]
+    NoStepKeyword(String),
+
+    /// Unknown scenario step keyword.
     ///
     /// Each scenario step must start with a known keyword (given,
     /// when, then, and, but), but Subplot didn't find one it
     /// recognized.
     ///
     /// This is usually due to a typing mistake or similar.
-    #[error("unknown step keyword")]
-    UnknownStepKind,
+    #[error("unknown step keyword: {0}")]
+    UnknownStepKind(String),
 
     /// Scenario step uses continuation keyword too early
     ///