Add possibility to create single index

This PR allows to create a single specific index. Note that `index-grep` allows this already. There are uses cases where `index-grep` does not work though. For instance, when I want to create `ch_bfs_arealstatistik-bodenbedeckung` then

`make index-grep IPATTERN=ch_bfs_arealstatistik-bodenbedeckung`

will create
`ch_bfs_arealstatistik-bodenbedeckung`, `ch_bfs_arealstatistik-bodenbedeckung-1985` and `ch_bfs_arealstatistik-bodenbedeckung-1997`, which is not what I want.

The new
`make index INDEX=ch_bfs_arealstatistik-bodenbedeckung`
will allow this use case.