String operators are not included in transform.py

[hhaawwaa]

Thanks for the work you are doing! Let me point that string operators such as:

• exact
• iexact
• contains
• icontains
• startswidth
• ... are not included in transform.py but are in Mongoengine. The same happens with geo operators

[hhaawwaa]

I am trying to implement it myself and make a pull request afterwards but not sure how to do it, I see motorengine code is quite different than mongoengine's. In any case, mongoengine uses regex to implement these operators as the following:

regex = r'%s'
if op == 'startswith':
    regex = r'^%s'
elif op == 'endswith':
    regex = r'%s$'
elif op == 'exact':
    regex = r'^%s$'

[hhaawwaa]

114 will include this feature