Add .lkml to LookML (#3454)

* Add .lkml to LookML

* Limit .lkml to .view.lkml and .model.lkml

* Add lkml samples

* Fix extension order

samples/LookML/example.model.lkml

@@ -0,0 +1,49 @@
+- label:          'desired label name'
+- connection:     connection_name
+- include:        filename_or_pattern
+  # Possibly more include declarations
+- persist_for:    N (seconds | minutes | hours)
+- case_sensitive: true | false
+- week_start_day: monday | tuesday | wednesday | thursday | friday | saturday | sunday
+- value_formats:
+  - name: desired_format_name
+    value_format: 'excel-style formatting string'
+  # Possibly more value formats
+
+- explore: view_name
+  label:  'desired label name'
+  description: 'description string'
+  symmetric_aggregates: true | false
+  hidden: true | false
+  fields: [field_or_set, field_or_set, …]
+
+  sql_always_where: SQL WHERE condition
+  always_filter:
+    field_name: 'looker filter expression'
+  conditionally_filter:
+    field_name: 'looker filter expression'
+    unless: [field_or_set, field_or_set, …]
+  access_filter_fields: [fully_scoped_field, fully_scoped_field, …]
+
+  always_join: [view_name, view_name, …]
+  joins:
+    - join: view_name
+      type: left_outer | full_outer | inner | cross
+      relationship: one_to_one | many_to_one | one_to_many | many_to_many
+      from: view_name
+      sql_table_name: table_name
+      view_label: 'desired label name'
+      fields: [field_or_set, field_or_set, …]
+      required_joins: [view_name, view_name, …]
+      foreign_key: dimension_name
+      sql_on: SQL ON clause
+    # Possibly more join declarations
+
+  persist_for: N (seconds | minutes | hours)
+  from: view_name
+  view: view_name
+  case_sensitive: true | false
+  sql_table_name: table_name
+  cancel_grouping_fields: [fully_scoped_field, fully_scoped_field, …]
+
+# Possibly more explore declarations