[export] Support exporting custom config variables

[archibate]

Related issue = #394

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This would allows user to specify configuration direct-in-place in Python.
I need this for TaichiHub, an online playground, for customizable particle numbers & screen size.
Please merge this soon, idealy before next release.

[Hanke98]

Hi Yubin, I think it's a really good idea to log some custom configurations and most of your code LGTM.

When I was testing the code, I wrote a simple test script:
`
import taichi as ti
ti.start_recording('test_record.yml')
ti.init(arch=ti.cc)
ti.record_action_config('num_particles', 8192)
n_particles = 8192

x = ti.Vector.field(2, float, n_particles)

@ti.kernel
def init():
for i in range(n_particles):
x[i] = [ti.random() * 0.4 + 0.2, ti.random() * 0.4 + 0.2]

init()
ti.stop_recording()
`

And I got:

[I 09/26/20 11:51:25.204] [action_recorder.cpp:start_recording@27] ActionRecorder: start recording to [test_record.yml]
[Taichi] Starting on arch=cc
Traceback (most recent call last):
File "test_record.py", line 4, in
ti.record_action_config('num_particles', 8192)
File "/home/ljf/playground/taichi/python/taichi/core/record.py", line 16, in record_action_config
record_action_entry('config', {'key': key, 'value': value})
File "/home/ljf/playground/taichi/python/taichi/core/record.py", line 6, in record_action_entry
ti_core.record_action_entry(name, list(contents.items()))
RuntimeError: Unexpected index

So, could you please have a look at that? Thank you very much!

[codecov]

Codecov Report

Merging #1893 into master will decrease coverage by 0.07%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1893      +/-   ##
==========================================
- Coverage   43.81%   43.73%   -0.08%     
==========================================
  Files          45       45              
  Lines        6174     6185      +11     
  Branches     1097     1099       +2     
==========================================
  Hits         2705     2705              
- Misses       3300     3311      +11     
  Partials      169      169              

Impacted Files	Coverage Δ
python/taichi/cc_compose.py	0.00% <0.00%> (ø)
python/taichi/core/record.py	0.00% <0.00%> (ø)

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[Hanke98]

Great! Thanks again! Learnt a lot from your code!