Styler class doesn't apply formatting from Styler.format when writing to Excel
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
df = pd.DataFrame({'a': [.1, .2], 'b': [.2, .3]})
df.style.background_gradient().format("{:.2%}").to_excel('test.xlsx')
```

or to prevent actually writing to disk:

```python
buf = StringIO()
df = pd.DataFrame({'a': [.1, .2], 'b': [.2, .3]})

df.style.background_gradient().format("{:.2%}".format).to_excel(buf)

buf.seek(0)
pd.read_excel(buf)
```

When reading the file the gradient will be applied correctly but the percentage formatting will not be.

#### Problem description

When the `Styler.to_excel` is called it doesn't write styles added using the `Styler.format` method. The desired behaviour should be that all styling and formatting associated with the `Styler` object should be written to the excel file just as it gets rendered in HTML.

The excel writer being used is `openpyxl: 2.5.3`


#### Expected Output

Excel document with formatting operations included.

#### Output of ``pd.show_versions()``


<details>

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.13.final.0
python-bits: 64
OS: Darwin
OS-release: 17.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: None.None

pandas: 0.23.0
pytest: None
pip: 9.0.3
setuptools: 39.0.1
Cython: None
numpy: 1.14.3
scipy: None
pyarrow: None
xarray: None
IPython: 5.7.0
sphinx: 1.7.4
patsy: None
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.2.2
openpyxl: 2.5.3
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 1.0.1
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>