foreign key with "_id" in the migrate.add_index
I cannot replicate this problem:

```python

from peewee import *

db = SqliteDatabase(':memory:')

class Base(Model):
    class Meta:
        database = db

class User(Base):
    username = TextField()

class Tweet(Base):
    user_id = ForeignKeyField(User, backref='tweets')

db.create_tables([User, Tweet])
db.execute_sql('drop index "tweet_user_id"')

from playhouse.migrate import *
migrator = SchemaMigrator.from_database(db)
migrate(
    migrator.add_index('tweet', ('user_id',), unique=True)
)
```

The SQL being generated is correct in the above example:

```sql
CREATE TABLE IF NOT EXISTS "user" ("id" INTEGER NOT NULL PRIMARY KEY, "username" TEXT NOT NULL)
CREATE TABLE IF NOT EXISTS "tweet" ("id" INTEGER NOT NULL PRIMARY KEY, "user_id" INTEGER NOT NULL, FOREIGN KEY ("user_id") REFERENCES "user" ("id"))
-- this is added automatically by peewee --
CREATE INDEX IF NOT EXISTS "tweet_user_id" ON "tweet" ("user_id")
-- we'll drop the automatically-added index explicitly --
drop index "tweet_user_id"

-- this is generated by the migrator --
CREATE UNIQUE INDEX "tweet_user_id" ON "tweet" ("user_id")
```

Also, for what it's worth, using "master_id" is a bit of an anti-pattern, since the field accessor returns an `AnotherThing` instance and not a numeric/unique ID.
I have a foreign key in the ORM object and in the database with name ending with '_id'. Like this:
```
class Thing(BaseThing):
    master_id = ForeignKeyField(AnotherThing, backref='details', index=True)
    other_field =  BigIntegerField(index=True)
    ...
```

and I'm trying to create an index in migration using add_index

Attempt 1
```
def migrate(migrator, database, fake=False, **kwargs):
    migrator.add_index('thing', 'master', 'other_field', unique=True)
```
fails with message: `peewee.InternalError: (1072, "Key column 'master' doesn't exist in table")`

Attempt 2
```
def migrate(migrator, database, fake=False, **kwargs):
    migrator.add_index('thing', 'master_id', 'other_field', unique=True)
```
fails with message: `peewee.InternalError: (1072, "Key column 'master_id_id' doesn't exist in table")`

Is it possible to make a `migrate.add_index` work in this situation?
