Convtools

[14](https://github.com/westandskif/convtools/issues/14)

**Misc**

- improved performance of ``Table.chain``, ``Table.into_iter_rows`` and
``Table.into_csv`` methods
- improved performance of ``c.apply_func``

[11](https://github.com/westandskif/convtools/pull/11)

**Features**

- added ``c.chunk_by(c.item("x"), size=100)`` for slicing iterables into chunks
by element values and/or size of chunk
- added ``c.chunk_by_condition(c.CHUNK.item(-1) - c.this() < 100)`` for slicing
iterables into chunks based on condition, which is a function of a current
chunk and a current element
- added ``(...).len()`` shortcut for ``c.call_func(len, c.this())``

**Misc**

- no longer create empty ``labels_`` dict on each converter call where no
labels are going to be used
- no longer create new ``This`` instances, now reusing an existing one

**Clean-up**

Normally you use ``c.ReduceFuncs.Sum(c.this())`` to reduce something, but it's
possible to use custom reduce functions like this:

* ``c.reduce(lambda x, y: x + y, c.this(), initial=0)``
* ``c.reduce(c.inline_expr("{} + {}"), c.this(), initial=0)``

``c.reduce`` used to support ``prepare_first`` parameter which was adding
confusion. Now it's dropped.

**Features**

[6](https://github.com/westandskif/convtools/issues/6)

- added ``c.take_while`` and ``(...).take_while`` re-implementation of
``itertools.takewhile``
- added ``c.drop_while`` and ``(...).drop_while`` re-implementation of
``itertools.dropwhile``

**Features**

- added ``Table.zip`` method to stitch tables (joining on row indexes)
- added ``Table.chain`` method to put tables together one after another

**Features**

- introduced ``Table`` conversions [3](https://github.com/westandskif/convtools/pull/3)
- added ``c.apply_func``, ``c.apply`` and ``(...).apply_method`` conversions

**Bugfix**

- fixed inner join with inner loop with soft conditions: any condition except
for ``==`` and ``c.and_``
- fixed piping to callable with further calling pipe methods like ``as_type``,
``filter`` and ``sort``

**Misc**

- reworked main converter callable wrapper so that it no longer dumps sources
onto disk for beautiful stacktraces when the converter returns a generator
(it used to make them down almost 2 times slower). If such debugging is
needed, just enable debug. As for simple exceptions, these still dump code to
disc on Exceptions because this should be cheap.