Add basic Lua ALE functions and test coverage

Ensure that basic ALE functions `ale.var`, `ale.escape`, and `ale.env`
are available in Lua. Cover all Lua code so far with busted tests,
fixing bugs where ALE variables can be set with Boolean values instead
of numbers. Document all functionality so far.

test/script/run-lua-tests

@@ -0,0 +1,57 @@
+#!/usr/bin/env bash
+
+set -e
+set -u
+
+docker_flags=(--rm -v "$PWD:/testplugin" -v "$PWD/test:/home" -w /testplugin/test/lua "$DOCKER_RUN_IMAGE")
+
+quiet=0
+
+while [ $# -ne 0 ]; do
+    case $1 in
+    -q)
+        quiet=1
+        shift
+    ;;
+    --)
+        shift
+        break
+    ;;
+    -?*)
+        echo "Invalid argument: $1" 1>&2
+        exit 1
+    ;;
+    *)
+        break
+    ;;
+    esac
+done
+
+function filter-busted-output() {
+    local hit_failure_line=0
+
+    while read -r; do
+        if ((quiet)); then
+            # If we're using the quiet flag, the filter out lines until we hit
+            # the first line with "Failure" and then print the rest.
+            if ((hit_failure_line)); then
+                echo "$REPLY"
+            elif [[ "$REPLY" = *'Failure'* ]]; then
+                hit_failure_line=1
+                echo "$REPLY"
+            fi
+        else
+            echo "$REPLY"
+        fi
+    done
+}
+
+exit_code=0
+
+set -o pipefail
+"$DOCKER" run -a stdout "${docker_flags[@]}" /usr/bin/busted-5.1 \
+    -m '../../lua/?.lua;../../lua/?/init.lua' . \
+    --output utfTerminal | filter-busted-output || exit_code=$?
+set +o pipefail
+
+exit "$exit_code"