add coverage handler spec

spec/coverage_output_handler.moon

@@ -0,0 +1,51 @@
+
+load_line_table = (chunk_name) ->
+  import to_lua from require "moonscript.base"
+
+  return unless chunk_name\match "^@"
+  fname = chunk_name\sub 2
+
+  file = assert io.open fname
+  code = file\read "*a"
+  file\close!
+
+  c, ltable = to_lua code
+
+  return nil, ltable unless c
+
+  line_tables = require "moonscript.line_tables"
+  line_tables[chunk_name] = ltable
+  true
+
+(options) ->
+  busted = require "busted"
+  handler = require("busted.outputHandlers.utfTerminal") options
+
+  local spec_name
+
+  coverage = require "moonscript.cmd.coverage"
+  cov = coverage.CodeCoverage!
+
+  busted.subscribe { "test", "start" }, (context) ->
+    cov\start!
+
+  busted.subscribe { "test", "end" }, ->
+    cov\stop!
+
+  busted.subscribe { "suite", "end" }, (context) ->
+    line_counts = {}
+
+    for chunk_name, counts in pairs cov.line_counts
+      continue unless chunk_name\match("^@$./") or chunk_name\match "@[^/]"
+      continue if chunk_name\match "^@spec/"
+
+      if chunk_name\match "%.lua$"
+        chunk_name = chunk_name\gsub "lua$", "moon"
+        continue unless load_line_table chunk_name
+
+      line_counts[chunk_name] = counts
+
+    cov.line_counts = line_counts
+    cov\format_results!
+
+  handler