Enable key placeholder to be combined with other data within the same command argument

[filipecosta90]

Fixes #153
This PR adds the option to combine key placeholders with other characters within the same argument. This is important for RedisGraph and RediSearch for us to be able to mix for example node ids with other conditions, etc...
I've used this feature on RG 2.0 benchmark and forgot to open the PR to kick off this feature discussion.
Example of usage:

memtier_benchmark -c 10 -t 1 \
    --command="graph.query GRAPH1 \"MATCH (s:vertex)-[*6]->(t) WHERE id_in_graph(s)=__key__ RETURN count(t)\"" \
    --key-maximum=100000 --distinct-client-seed --command-key-pattern G --key-prefix=""

This PR includes two extra tests