Add custom field name option for Cards

[ImplyingICheck]

Allow ability to set specific field_names. That is, the ability to name Field3 to "Term" and Field4 to "Definition" and leave the rest of the fields as Field1, Field2, ..., FieldN

This involves refactoring the way Card is constructed

[ankideckconstructor] card.py (Lines 11-12)

  def __init__(self, fields, has_html=False, tags=None, field_names=None,
               note_type=None, deck=None, guid=None):

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and changing the logic for setting generic field names

[ankideckconstructor] card.py (Line 22)

 def __set_field_names(field_names, n_fields):

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^{Created from JetBrains using CodeStream}

[ImplyingICheck]

Implemented by [#25].

As of [#35], the generic field_names are of form Field<idx> where "" is the corresponding column of the tab separated value file.

Any value which evaluates as False can be used to designate a generic file name. For example:

['', False, 'Some Wanted Name'] #=> ['Field0', 'Field1', 'Some Wanted Name']