Add support for placeholders in migration files

The placeholder prefix, suffix and key-value pairs can be specificed in
application.conf, e.g.

```
db.default.migration.placeholderPrefix="$flyway{{{"
db.default.migration.placeholderSuffix="}}}"
db.default.migration.placeholders.foo="bar"
db.default.migration.placeholders.hoge="pupi"
```

This would cause

```
INSERT INTO USERS ($flyway{{{foo}}}) VALUES ('$flyway{{{hoge}}}')
```

to be rewritten to

```
INSERT INTO USERS (bar) VALUES ('pupi')
```
Merge pull request #22 from cb372/feature/custom-placeholders

Add support for placeholders in migration files