Implemented str * int operations

[nouritsu]

Feature Addition -
str * int will result in a string that is repeated int times
If int is negative or zero, resulting value will be null.

[nouritsu]

Question: Where do I add tests? I could not find a test for multiplication and division in the jaq-core/tests/ directory.

However, I have tested the implementation on the command line and it works as expected -

> jaq -n '5 * \"hello\"' 
"hellohellohellohellohello"

> jaq -n '\"hello\" * 5' 
"hellohellohellohellohello"

> jaq -n '\"hello\" * -5'
null

> jaq -n '\"hello\" * 0'
null

This indicates that the commutative property is satisfied and the results obtained are the exact same as on the latest version of jq.

> jq -n '5 * \"hello\"' 
"hellohellohellohellohello"

> jq -n '\"hello\" * 5' 
"hellohellohellohellohello"

> jq -n '\"hello\" * -5'
null

> jq -n '\"hello\" * 0'
null

[nouritsu]

Tests have been added, this is ready to merge.
#71

[01mf02]

Thanks a lot!