Add support to environment variable define the default limit

[Claudson Oliveira]

Based on #33 and to avoid always to write the limit word we could to have a environment variable that defines that
eg.: GITQL_DEFAULT_LIMIT

[Luiz Fernando Peres]

How about passing that on a flag as well?

[Claudson Oliveira]

@luizperes for me there is no difference between use the query parameter or a flag.

[Claudson Oliveira]

For me it's something that we can think better in the future, I'm leaving the idea to keep the issues more updated.