M2m accessors

[jawadst]

This PR for #232 allows using an accessor (as) when defining a many-to-many relationship while keeping the value in the original field to behave the same as ForeignKey with as.

Example definition:

User.fields = {
    id: attr(),
    projects_id: many({ to: 'Project', relatedName: 'users', as: 'projects' }),
};

which allows accessing the related projects with:

session.User.create({ id: 'u0', projects_id: ['p0', 'p1'] });

session.User.withId('u0').projects // QuerySet<Project>
session.User.withId('u0').projects_id // Array of IDs (['p0', 'p1'])

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The PR contains the following modifications:

• Pass as to the virtual field of the M2M relationship so that we can populate the right field when refreshing the M2M values
• Update the QuerySet on the field named as instead of the original field name if configured
• Keep the original field from props as is in create/update when an accessor is configured
• Update to the README to add an example of a fk and many with an accessor

[codecov-io]

Codecov Report

Merging #279 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #279      +/-   ##
==========================================
+ Coverage   99.81%   99.81%   +<.01%     
==========================================
  Files          14       14              
  Lines        1080     1082       +2     
  Branches      214      218       +4     
==========================================
+ Hits         1078     1080       +2     
  Misses          2        2

Impacted Files	Coverage Δ
src/fields.js	100% <ø> (ø)	⬆️
src/Model.js	100% <100%> (ø)	⬆️

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[haveyaseen]

Wow, this is a perfect PR! Thank you so much for considering even the edge cases. I'll also add a test for the update check and add a little whitespace.

[jawadst]

@haveyaseen Thanks for reviewing and completing my PR!