Allow TextClause in Index __init__ (#99)

Quick fix that allow support for: 

```python
# From official doc
from sqlalchemy import text

Table("sometable", metadata,
                Column("name", String(50)),
                Column("address", String(100)),
                Index("some_index", text("lower(name)"))
        )
```

Author: isra17

sqlalchemy-stubs/sql/schema.pyi

@@ -4,7 +4,7 @@ from typing import (
 )
 from . import visitors, functions
 from .base import SchemaEventTarget as SchemaEventTarget, DialectKWArgs as DialectKWArgs, ColumnCollection
-from .elements import ColumnClause as ColumnClause
+from .elements import ColumnClause as ColumnClause, TextClause
 from .selectable import TableClause as TableClause
 from .type_api import TypeEngine
 from .. import util
@@ -304,8 +304,8 @@ class Index(DialectKWArgs, ColumnCollectionMixin, SchemaItem):
     name: str = ...
     unique: bool = ...
     info: Optional[Mapping[str, Any]] = ...
-    def __init__(self, name: str, *expressions: Union[Column[Any], str], unique: bool = ..., quote: Optional[bool] = ...,
-                 info: Optional[Mapping[str, Any]] = ..., **kw: Any) -> None: ...
+    def __init__(self, name: str, *expressions: Union[TextClause, Column[Any], str], unique: bool = ...,
+                 quote: Optional[bool] = ..., info: Optional[Mapping[str, Any]] = ..., **kw: Any) -> None: ...
     @property
     def bind(self) -> Optional[Union[Engine, Connection]]: ...
     def create(self: _I, bind: Optional[Union[Engine, Connection]] = ...) -> _I: ...

test/test-data/sqlalchemy-sql-schema.test

@@ -60,3 +60,17 @@ class Mytable(Base):
     __tablename__ = 'mytable'
     objid = Column(ForeignKey('othertable.objid'), index=True)
 [out]
+
+[case testTableWithIndexes]
+from sqlalchemy import Column, Table, String, Integer, Index, MetaData, text, func
+metadata = MetaData()
+name_col = Column('name', String)
+test_table = Table('test', metadata,
+    Column('id', Integer, primary_key=True),
+    name_col,
+    Index('idx1', 'id', 'name'),
+    Index('idx1', 'id', name_col),
+    Index('idx1', 'id', func.lower(name_col)),
+    Index('idx1', 'id', text("lower(name)")),
+)
+[out]