[GH-116] Add ichimoku cloud width

[jealous]

The Ichimoku Cloud is a collection of technical indicators that show support and resistance levels, as well as momentum and trend direction.

In this implementation, we only calculate the delta between lead A and lead B (which is the width of the cloud).

It contains three windows:

• window for the conversion line, default to 9
• window for the baseline and the shifts, default to 26
• window for the leading line, default to 52

Formular:

• conversion line = (PH9 + PL9) / 2
• baseline = (PH26 + PL26) / 2
• leading span A = (conversion line + baseline) / 2
• leading span B = (PH52 + PL52) / 2
• result = leading span A - leading span B

Where:

• PH = Period High
• PL = Period Low

Examples:

• df['ichimoku'] returns the ichimoku cloud width with default windows
• df['ichimoku_7,22,44'] returns the ichimoku cloud width with window sizes 7, 22, 44

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.03 :tada:

Comparison is base (fc0a86d) 98.57% compared to head (57700c7) 98.61%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #147 +/- ## ========================================== + Coverage 98.57% 98.61% +0.03% ========================================== Files 1 1 Lines 841 864 +23 ========================================== + Hits 829 852 +23 Misses 12 12 ``` | [Impacted Files](https://app.codecov.io/gh/jealous/stockstats/pull/147?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Cedric+Zhuang) | Coverage Δ | | |---|---|---| | [stockstats.py](https://app.codecov.io/gh/jealous/stockstats/pull/147?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Cedric+Zhuang#diff-c3RvY2tzdGF0cy5weQ==) | `98.61% <100.00%> (+0.03%)` | :arrow_up: |

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